Methods and devices for fabricating and assembling printable semiconductor elements

ABSTRACT

The invention provides methods and devices for fabricating printable semiconductor elements and assembling printable semiconductor elements onto substrate surfaces. Methods, devices and device components of the present invention are capable of generating a wide range of flexible electronic and optoelectronic devices and arrays of devices on substrates comprising polymeric materials. The present invention also provides stretchable semiconductor structures and stretchable electronic devices capable of good performance in stretched configurations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/640,206, filed Jun. 30, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/084,211, now U.S. Pat. No. 9,768,086, and U.S.patent application Ser. No. 15/084,091, now U.S. Pat. No. 9,761,444,both filed Mar. 29, 2016, which are continuations of U.S. patentapplication Ser. No. 14/155,010, filed Jan. 14, 2014, now U.S. Pat. No.9,450,043, which is a continuation of U.S. patent application Ser. No.13/801,868, filed Mar. 13, 2013, now U.S. Pat. No. 8,664,699, which is acontinuation of U.S. patent application Ser. No. 13/113,504, filed onMay 23, 2011, now U.S. Pat. No. 8,440,546, which is a continuation ofU.S. patent application Ser. No. 12/564,566, filed on Sep. 22, 2009, nowU.S. Pat. No. 7,982,296, which is a division of U.S. Nonprovisionalpatent application Ser. No. 11/145,574, filed Jun. 2, 2005, now U.S.Pat. No. 7,622,367, which claims priority under 35 U.S.C. 119(e) to U.S.Provisional Patent Application Nos. 60/577,077, 60/601,061, 60/650,305,60/663,391 and 60/677,617, filed on Jun. 4, 2004, Aug. 11, 2004, Feb. 4,2005, Mar. 18, 2005, and May 4, 2005, respectively, all of which arehereby incorporated by reference in their entireties to the extent notinconsistent with the disclosure herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made, at least in part, with United Statesgovernmental support awarded by Department of Energy under Grant No.DEFG02-91ER45439 and the Defense Advanced Projects Agency under ContractF8650-04-C-710. The United States Government has certain rights in thisinvention.

BACKGROUND OF INVENTION

Since the first demonstration of a printed, all polymer transistor in1994, a great deal of interest has been directed at a potential newclass of electronic systems comprising flexible integrated electronicdevices on plastic substrates. [Garnier, F., Hajlaoui, R., Yassar, A.and Srivastava, P., Science, Vol. 265, pgs 1684-1686] Recently,substantial research has been directed toward developing new solutionprocessable materials for conductors, dielectrics and semiconductorselements for flexible plastic electronic devices. Progress in the fieldof flexible electronics, however, is not only driven by the developmentof new solution processable materials but also by new device componentgeometries, efficient device and device component processing methods andhigh resolution patterning techniques applicable to plastic substrates.It is expected that such materials, device configurations andfabrication methods will play an essential role in the rapidly emergingnew class of flexible integrated electronic devices, systems andcircuits.

Interest in the field of flexible electronics principally arises out ofseveral important advantages provided by this technology. First, themechanical ruggedness of plastic substrate materials provides electronicdevices less susceptible to damage and/or electronic performancedegradation caused by mechanical stress. Second, the inherentflexibility of these substrate materials allows them to be integratedinto many shapes providing for a large number of useful deviceconfigurations not possible with brittle conventional silicon basedelectronic devices. For example, bendable flexible electronic devicesare expected to enable fabrication of new devices, such as electronicpaper, wearable computers and large-area high resolution displays, thatare not easily achieved with established silicon based technologies.Finally, the combination of solution processable component materials andplastic substrates enables fabrication by continuous, high speed,printing techniques capable of generating electronic devices over largesubstrate areas at low cost.

The design and fabrication of flexible electronic devices exhibitinggood electronic performance, however, present a number of significantchallenges. First, the well developed methods of making conventionalsilicon based electronic devices are incompatible with most plasticmaterials. For example, traditional high quality inorganic semiconductorcomponents, such as single crystalline silicon or germaniumsemiconductors, are typically processed by growing thin films attemperatures (>1000 degrees Celsius) that significantly exceed themelting or decomposition temperatures of most plastic substrates. Inaddition, most inorganic semiconductors are not intrinsically soluble inconvenient solvents that would allow for solution based processing anddelivery. Second, although many amorphous silicon, organic or hybridorganic-inorganic semiconductors are compatible with incorporation intoplastic substrates and can be processed at relatively low temperatures,these materials do not have electronic properties capable of providingintegrated electronic devices capable of good electronic performance.For example, thin film transistors having semiconductor elements made ofthese materials exhibit field effect mobilities approximately threeorders of magnitude less than complementary single crystalline siliconbased devices. As a result of these limitations, flexible electronicdevices are presently limited to specific applications not requiringhigh performance, such as use in switching elements for active matrixflat panel displays with non-emissive pixels and in light emittingdiodes.

Progress has recently been made in extending the electronic performancecapabilities of integrated electronic devices on plastic substrates toexpand their applicability to a wider range of electronics applications.For example, several new thin film transistor (TFT) designs have emergedthat are compatible with processing on plastic substrate materials andexhibit significantly higher device performance characteristics thanthin film transistors having amorphous silicon, organic or hybridorganic-inorganic semiconductor elements. One class of higher performingflexible electronic devices is based on polycrystalline silicon thinfilm semiconductor elements fabricated by pulse laser annealing ofamorphous silicon thin films. While this class of flexible electronicdevices provides enhanced device electronic performance characteristics,use of pulsed laser annealing limits the ease and flexibility offabrication of such devices, thereby significantly increasing costs.Another promising new class of higher performing flexible electronicdevices is devices that employ solution processable nanoscale materials,such as nanowires, nanoribbons, nanoparticles and carbon nanotubes, asactive functional components in a number of macroelectronic andmicroelectronic devices.

Use of discrete single crystalline nanowires or nanoribbons has beenevaluated as a possible means of providing printable electronic deviceson plastic substrates that exhibit enhanced device performancecharacteristics. Duan et al. describe thin film transistor designshaving a plurality of selectively oriented single crystalline siliconnanowires or CdS nanoribbons as semiconducting channels [Duan, X., Niu,C., Sahl, V., Chen, J., Parce, J., Empedocles, S. and Goldman, J.,Nature, Vol. 425, pgs, 274-278]. The authors report a fabricationprocess allegedly compatible with solution processing on plasticsubstrates in which single crystalline silicon nanowires or CdSnanoribbons having thicknesses less than or equal to 150 nanometers aredispersed into solution and assembled onto the surface of a substrateusing flow-directed alignment methods to produce the semiconductingelement of at thin film transistor. An optical micrograph provided bythe authors suggests that the disclosed fabrication process prepares amonolayer of nanowires or nanoribbons in a substantially parallelorientation and spaced apart by about 500 nanometers to about 1,000nanometers. Although the authors report relatively high intrinsic fieldaffect mobilities for individual nanowires or nanoribbons 119 cm² V⁻¹s⁻¹), the overall device field effect mobility has recently beendetermined to be “approximately two orders of magnitude smaller” thanthe intrinsic field affect mobility value reported by Duan et al.[Mitzi, D. B, Kosbar, L. L., Murray, C. E., Copel, M. Afzali, A.,Nature, Vol. 428, pgs. 299-303]. This device field effect mobility isseveral orders of magnitude lower than the device field effectmobilities of conventional single crystalline inorganic thin filmtransistors, and is likely due to practical challenges in aligning,densely packing and electrically contacting discrete nanowires ornanoribbons using the methods and device configurations disclosed inDuan et al.

Use of a nanocrystal solutions as precursors to polycrystallineinorganic semiconductor thin films has also been explored as a possiblemeans of providing printable electronic devices on plastic substratesthat exhibit higher device performance characteristics. Ridley et al.disclose a solution processing fabrication method wherein a solutioncadmium selenide nanocrystals having dimensions of about 2 nanometers isprocessed at plastic compatible temperatures to provide a semiconductorelement for a field effect transistor. The authors report a methodwherein low temperature grain growth in a nanocrystal solution ofcadmium selenide provides single crystal areas encompassing hundreds ofnanocrystals. Although Ridley et al. report improved electricalproperties relative to comparable devices having organic semiconductorelements, the device mobilities achieved by these techniques (≈1 cm² V⁻¹s⁻¹) are several orders of magnitude lower than the device field effectmobilities of conventional single crystalline inorganic thin filmtransistors. Limits on the field effect mobilities achieved by thedevice configurations and fabrication methods of Ridley et al. arelikely to arise from the electrical contact established betweenindividual nanoparticles. Particularly, the use of organic end groups tostabilize nanocrystal solutions and prevent agglomeration may impedeestablishing good electrical contact between adjacent nanoparticles thatis necessary for providing high device field effect mobilities.

Although Duan et al. and Ridley et al. provide methods for fabricatingthin film transistors on plastic substrates, the device configurationsdescribed employ transistors comprising mechanically rigid devicecomponents, such as electrodes, semiconductors and/or dielectrics.Selection of a plastic substrate with good mechanical properties mayprovide electronic devices capable of performing in flexed or distortedorientations. However, such motion is expected to generate mechanicalstrain on the individual rigid transistor device components. Thismechanical strain may induce damage to individual components, forexample by cracking, and also may degrade or disrupt electrical contactbetween device components.

It will be appreciated from the foregoing that there is currently a needin the art for methods and device configurations for fabricatingintegrated electronic semiconductor-containing devices on plasticsubstrates. Printable semiconductor elements having good electricalcharacteristics are needed to allow effective device fabrication attemperatures compatible with assembly on plastic polymer substrates. Inaddition, methods of printing semiconductor materials onto large areasof plastic substrates are needed to enable continuous, high speedprinting of complex integrated electrical circuits over large substrateareas. Finally, fully flexible electronic devices capable of goodelectronic performance in flexed or deformed device orientations areneeded to enable a wide range of new flexible electronic devices.

SUMMARY OF THE INVENTION

The present invention provides methods, devices and device componentsfor fabricating structures and/or devices, such assemiconductor-containing electronic devices, on substrate surfaces, suchas plastic substrates. Specifically, the present invention providesprintable semiconductor elements for fabricating electronic devices,optoelectronic devices and other functional electronic assemblies byflexible, low cost printing methods. It is an object of the presentinvention to provide methods and devices for fabricating semiconductorelements, such as unitary single crystalline inorganic semiconductorshaving selected physical dimensions ranging from about 10 s ofnanometers to about 10 s of centimeters, which are capable of highprecision assembly on substrate surfaces via a range of printingtechniques. It is another object of the present invention to providemethods for assembling and/or patterning printable semiconductorelements using dry transfer contact printing and/or solution printingtechniques which provide good placement accuracy and pattern fidelityover large substrate areas. It is further an object of the presentinvention to provide good electronic performance integrated electronicand/or optoelectronic devices comprising one or more printablesemiconductor elements supported by a plastic substrate, particularlyfully flexible thin film transistors having printable semiconductorelements exhibiting good electronic performance characteristics, such asfield effect mobilities, threshold voltages and on-off ratios.

In one aspect the present invention provides methods of fabricating highperformance electronic and/or optoelectronic devices or devicecomponents having one or more printable components, such as a printablesemiconductor element. Electronic and optoelectronic devices which maybe fabricated by the methods of the present invention, include but arelimited to, transistors, diodes, light emitting diodes (LEDS), lasers,organic light emitting diodes (OLEDS), microelectromechanical systems(MEMS) and nanoelectromechanical systems (NEMS). Particularly, thepresent invention provides methods of assembling semiconductor elementsand/or other device components via printing techniques into electronicand/or optoelectronic devices or device components which exhibitperformance characteristics comparable to single crystallinesemiconductor based devices fabricated by conventional high temperatureprocessing methods.

In an embodiment of the present invention useful for device fabricationon substrates having low melting or decomposition temperatures, such asplastic substrates and semiconductor substrates, methods of the presentinvention comprise independently performable fabrication steps of: (1)forming one or more discrete, high quality semiconductor elements and(2) assembling and/or patterning these semiconductor elements and otherdevice components on a substrate surface. For example, the presentinvention includes methods wherein independent, high quality printableinorganic semiconductors are generated by masking and etching bulksingle crystalline inorganic semiconductor materials fabricated byconventional high temperature processing methods, such as hightemperature (>1000 Celsius) film growth, doping and other processingtechniques. After fabrication, such printable inorganic semiconductorsare assembled onto one or more substrates surface by printing techniqueswhich may be performed at relatively low temperatures (<about 400degrees Celsius). An advantage of having independently performablepreparation and patterning/assembly steps is that each step may beperformed at ambient conditions, such as temperatures and ambientcontamination levels (i.e. if clean room conditions are needed), whichoptimize the efficiencies, flexibilities and utilities of eachindependently performable fabrication step. For example, the presentmethods allow semiconductor materials to be fabricated at the hightemperatures needed to generate high quality single crystallinesemiconductors. Semiconductor element patterning and/or assembly,however, may be subsequently carried out at substantially lowertemperatures favorable for device fabrication on substrates having lowmelting or decomposition temperatures, such as plastic substrates. Inthis manner, high performance devices may be fabricated on a wide rangeof substrate surfaces without significant melting, decomposition ordamage to the substrate surface. Another advantage of separatingsemiconductor fabrication from semiconductor/device assembly is thatintegration of the semiconductor elements into high performance devicesand device components may be achieved by a wide range of low cost andflexible assembly methods, such as dry transfer and solution printingtechniques, which do not require clean room conditions and arecompatible with continuous, high speed device fabrication on large areasof substrates. In the context of this aspect of the present invention,the present methods are compatible with printing on substratescomprising virtually any material, including plastic substrates andnon-plastic substrates, such as semiconductor wafers, for examplesilicon wafers or GaAs wafers.

In another aspect, the present invention provides printablesemiconductor elements for integration into high performance electricaland optoelectronic devices and device components. In the context of thepresent invention, the term “printable” relates to materials,structures, device components and/or integrated functional devices thatmay be transferred, assembled, patterned, organized and and/orintegrated onto or into substrates without exposure of the substrate tohigh temperatures (i.e. at temperatures less than or equal to about 400degrees Celsius). Printable semiconductors of the present invention maycomprise semiconductor structures that are able to be assembled and/orintegrated onto substrate surfaces by dry transfer contact printingand/or solution printing methods. Exemplary semiconductor elements ofthe present invention may be fabricated by “top down” processing of arange of inorganic semiconductor materials including, but not limitedto, single crystalline silicon wafers, silicon on insulator wafers,polycrystalline silicon wafers and GaAs wafers. Printable semiconductorelements derived from high quality semiconductor wafers, for examplesemiconductor wafers generated using conventional high temperature vapordeposition processing techniques, are beneficial for applicationsrequiring good electronic performance because these materials havebetter purities and extents of crystallization than materials preparedusing “bottom up” processing techniques, such as conventional techniquesfor making nanocrystals and nanowires. Another advantage provided by the“top-down” processing methods of the present invention is that printablesemiconductor elements and arrays of printable semiconductor elementsmay be fabricated in well defined orientations and patterns, unlike“bottom-up” processing methods typically used for fabricating nanowiresand nanoparticles. For example, semiconductor elements may be fabricatedin arrays having positions and spatial orientations directlycorresponding to the eventual positions and spatial orientations ofthese elements in functional devices or arrays of functional devices,such as transistor arrays or diode arrays.

Printable semiconductor elements may comprise unitary, singlecrystalline inorganic semiconductor structures having wide range ofshapes, such as ribbon (or strips), discs, platelets, blocks, post,cylinders or any combinations of these shapes. Printable semiconductorelements of the present invention may have a wide range of physicaldimensions, for example, thicknesses ranging from about 10 nanometers toabout 100 microns, widths ranging from about 50 nanometers to about 1millimeter and lengths ranging from about 1 micron to about 1millimeter. Use of semiconductor elements having thicknesses greaterthan about 10 nanometers and widths greater than about 500 nanometer arepreferred for some application because these dimensions may provideelectronic devices exhibiting good electronic performance, such as thinfilm transistors having a device field effect mobility greater than orequal to about 100 cm² V⁻¹ s⁻¹, and preferably greater than or equal toabout 300 cm² V⁻¹ s⁻¹ and more preferably greater than or equal to about800 cm² V⁻¹ s⁻¹. In addition, semiconductor elements having widthsgreater than about 10 nanometers can be assembled on substrates by arange of printing techniques with good placement accuracy and patternfidelity.

Printable semiconductor elements of the present invention may also beprovided with an alignment maintaining element that mechanicallyconnects the printable semiconductor element to a mother substrate, suchas a semiconductor wafer. Alignment maintaining elements are useful formaintaining a selected orientation and/or position of a printablesemiconductor element during transfer, assembly and/or integrationprocessing steps. Alignment maintaining elements are also useful formaintaining relative positions and orientations of a plurality ofsemiconductor elements defining a selected pattern of semiconductorelements during transfer, assembly and/or integration processing steps.In methods of the present invention, alignment maintaining elementspreserve selected positions and orientations during contact (andbonding) of the printable semiconductor elements with the contactsurface of a conformable transfer device. Useful alignment maintainingelements in this aspect of the present invention are capable ofdisengaging from the printable semiconductor elements upon movement ofthe conformable transfer device without significantly changing theselected positions and orientations of the printable semiconductorelements. Disengagement is typically achieved by fracture or release ofthe alignment maintaining elements during movement of the transferdevice.

In one embodiment of the present invention, the printable semiconductorelement has a peanut shape characterized by wider ends and a narrowcentral region. In this embodiment, alignment maintaining elements areprovided via incomplete isotropic etching beneath the wider ends andcomplete isotropic etching beneath the central region. This processinglead to a semiconductor element connected to a mother substrate a twopoints corresponding to each end of the semiconductor element. Inanother embodiment, the printable semiconductor element has a ribbonshape extending along a central longitudinal axis. In this embodiment,alignment maintaining elements connect the both ends of the ribbon alongthe longitudinal axis to the mother substrate. In each embodiment,binding of the ribbon shaped or peanut shaped semiconductor element tothe contact surface of a transfer device and movement of the transferdevice results in fracture of both alignment maintaining elements andrelease of the printable semiconductor element from the mothersubstrate.

Printable semiconductor elements of the present invention haveindependently selectable physical dimensions, such as widths, heights,thicknesses surface roughness, and flatness, that are selectable with ahigh degree of accuracy. In an exemplary embodiment, physical dimensionsof printable semiconductor elements may be selected to within less thanabout 5%. Large numbers of printable semiconductor elements havinghighly uniform selected physical dimension may be fabricated using themethods of the present invention. In an exemplary embodiment, largenumbers of printable semiconductor elements can be fabricated havingphysical dimensions that vary by less than about 1%. The presentinvention, therefore, provides printable semiconductor elements withouta significant size and shape distributions, in contrast to conventionalmethods of generating nanowires. A significant advantage of thisapproach is that structures and devices that integrate printablesemiconductor elements of the present invention do not need to be builtto tolerate dispersion in size and shape of the semiconductor elements.In some embodiments, printable semiconductor elements of the presentinvention have very low surface roughness, for example having a surfaceroughness less than about 0.5 nanometers root mean square. Printablesemiconductor elements of the present invention may have one or moreflat surfaces. This configuration is beneficial in some devicefabrication applications because flat surfaces are useful forestablishing interfaces with other device components, such asconducting, semiconducting and/or dielectric device components.

Further, the present methods and compositions of matter provideprintable semiconductor elements comprising high quality semiconductormaterials. In some embodiments useful for fabricating high performanceelectrical devices, printable semiconductor elements have a purity witha factor of about 1000 or less of conventional semiconductor wafermaterials fabricated via high temperature processing techniques. Forexample, the present invention provides high purity semiconductorelements having oxygen impurities less than about 5 to 25 parts permillion atoms, carbon impurities less than about 1 to 5 parts permillion atoms, and heavy metal impurities less than or equal to about 1part per million atoms (ppma), preferably less than or equal to about100 parts per billion atoms (ppba) for some applications, and morepreferably less than or equal to about 1 part per billion atoms (ppba)for some applications. Printable semiconductor elements having lowlevels of heavy metal impurities (e.g. less than about 1 parts permillion atoms) are beneficial for applications and devices requiringgood electronic performance, as the presence of heavy metals insemiconductor materials can severely degrade their electricalproperties.

In addition, printable semiconductor elements of some aspects of thepresent invention have a very low resistivity gradient, for example aless than about 5% to 10% variation across their areas. This aspect ofthe present invention provides enhanced doping uniformity with respectto conventional semiconductor materials derived from “bottom up”processing techniques, such as nanowire and nanocrystal materials.Further, printable semiconductor elements of the present invention maycomprise semiconductor materials exhibiting very few dislocations, forexample less than 500 dislocations per square centimeter. Use ofsemiconductor elements comprising high quality semiconductor materialsis beneficial for device fabrication applications requiring goodelectronic performance.

In addition, the present methods and compositions of matter provideprintable semiconductor elements having highly uniform compositions. Inthis context, uniform composition refers to piece-to-piece uniformitywith respect to purity, dopant concentrations, dopant spatialdistributions and extents of crystallization. The high purities and gooduniformity with respect to the composition of printable semiconductorelements of the present provide functional devices exhibiting enhancedreliability with respect to devices fabricated from conventionalsemiconductor materials derived from “bottom up” processing techniques,such as nanowire and nanocrystal materials.

Printable semiconductor elements of the present invention preferablyhave at least one smooth surface, such as the top or bottom surface of amicroribbon, preferably exhibiting deviations from average surfaceposition of less than 10 nanometers, and more preferably for someapplications exhibiting deviations from average surface position of lessthan 1 Angstrom. Smooth surfaces of printable semiconductor elements ofthe present invention allow effective electrical contact and/or physicalintegration to be established with other device components in anintegrated electronic device or optoelectronic device.

Alternatively, printable semiconductor elements of the present inventionmay comprise composite semiconductor elements having a semiconductorstructure operationally connected to one or more additional structures,such as dielectric structures, conducting structures (e.g. electrode),additional semiconductor structures or any combination of these.Printable composite semiconductor elements provide materials and devicecomponents that may be easily and effectively integrated into complexelectronic or optoelectronic devices. In addition, the assembly methodsof the present invention allow printable semiconductor elements to beprovided in array geometries wherein adjacent elements are close to eachother, for example within 100 nanometers to 1 micron of each other. Forexample, a printable semiconductor element of the present inventioncomprises a unitary structure having a high quality semiconductorstructure, such as a single crystalline inorganic semiconductor,operationally connected to an inorganic dielectric structure, such as asilicon oxide layer. This embodiment of the present invention isparticular useful for fabrication of high performance thin filmtransistors because semiconductor and dielectric components may beassembled in a single printing step, and because use of unitarystructures comprising both semiconductor and dielectric componentsresults in insulator configurations exhibiting very low leakage ofelectric current from gate electrode to the semiconductor element orsource and drain electrodes. In another embodiment, printablesemiconductor elements of the present invention may comprise integratedfunctional devices, such as diodes, LEDS, transistors and OLEDS, whichmay be easily incorporated onto substrate surfaces.

The methods and compositions of the present invention provide aprocessing platform enabling fabrication of functional devicesexhibiting enhanced reliability with respect to devices based onsemiconductor materials generated via “bottom up” processing techniques,such as nanowires and nanocrystals. In this context, reliability refersto the capability of a functional devices to exhibit good electronicproperties over extended operating periods and refers to piece-to-pieceuniformity with respect to electrical properties of an ensemble ofdevice fabricated using the present methods and compositions. Forexample, devices of the present invention exhibit very uniform thresholdvoltages (e.g. standard deviation of less than 0.08V) and very uniformdevice mobilities (e.g. standard deviation of less than about 13%). Thisrepresents improvements in uniformities of threshold voltages and devicemobilities of a factor of about 40 and a factor of about 8,respectively, over nanowire based devices. The exceptional reliabilityof functional devices of the present invention is provided, at least inpart, by the high degree of uniformity of the compositions and physicaldimensions accessible using printable semiconductor elements of thepresent invention.

In another aspect, the present invention provides an electrical devicecomprising a first electrode, a second electrode and a printablesemiconductor element positioned in electrical contact with said firstand second electrodes. In one embodiment useful for applicationsrequiring good electrical device performance, the printablesemiconductor element comprises a unitary inorganic semiconductorstructure having physical dimensions and shape that provides a fillfactor between said first and second electrodes greater than or equal toabout 20%, preferably greater than or equal to about 50% for someapplications and more preferably greater than or equal to about 80% forsome applications. Optionally, electrically devices of this embodimentmay further comprise additional printable semiconductor elements, forexample printable elements substantially longitudinally oriented, andoptionally not in physical contact with each other. Importantly,multiple printable semiconductor elements of the present invention maybe configured in devices or device arrays in a manner providing largefill factors (e.g. greater than or equal to 20%, 50% or 80%) and goodelectronic performance, in contrast to systems involving densely packednanowire arrays. In one embodiment, the printable semiconductor elementhas at least one cross sectional dimension greater than or equal toabout 500 nanometers. In one embodiment, the printable semiconductorelement has a ratio of length to width equal to or less than about 10,preferably equal to or less than about 1.5 in for some applications. Inone embodiment, the printable semiconductor element has a ratio ofthickness to width equal to or less than about 0.1, preferably equal toor less than about 0.01 for some applications.

This aspect of the present invention further comprises arrays ofelectrical devices, such as transistors, diodes, photovoltaic devices,light emitting devices, comprising a first electrode, a second electrodeand a plurality printable semiconductor elements positioned inelectrical contact with the first and second electrode. In oneembodiment, an array of electrical devices comprises over 20 printablesemiconductor elements, preferably over 50 printable semiconductorelements for some applications and, more preferably over 100 printablesemiconductor elements for some applications. In one embodiment usefulfor applications requiring good electrical device performance, theprintable semiconductor elements provide a fill factor between saidfirst and second electrodes greater than or equal to about 20%,preferably greater than or equal to about 50% for some applications andmore preferably greater than or equal to about 80% for someapplications. Printable semiconductor elements may be substantiallylongitudinally oriented with respect to a selected alignment axis, suchas a selected alignment axis that extends along an axis which connectsthe closest points of said first and second electrical contacts. In oneembodiment, the relative positions and orientations of the printablesemiconductor elements are selected to within less than or equal toabout 5 microns. In one embodiment providing good end to endregistration of the semiconductor elements, each of said printablesemiconductor elements extends a length and terminates in first andsecond ends. In this embodiment, the first ends of said printablesemiconductor elements are positioned within 5 microns of the firstelectrode and the second ends of said printable semiconductor elementsare positioned within 5 microns of said second electrode. In oneembodiment, an array of electrical devices of the present inventioncomprises a plurality of printable semiconductors are provided in aconfiguration such that they are substantially longitudinally oriented,are not in physical contact with each other (i.e. do not overlap), andare in electrical contact with first and second electrodes. In oneembodiment, at least one physical dimension, such as average length,average width and/or average thickness, of the printable semiconductorelements in the array of electrical devices varies by less than about10%, preferably less than about 5% for some applications. In thisembodiment, the printable semiconductor elements in the array haveselected physical dimensions, such as average lengths, average widthsand/or average thickness, that do not vary significantly (i.e. less thanabout 10%) from each other.

In another aspect, the present invention provides a transistor having aprintable semiconductor element. In one embodiment, a transistor of thepresent invention comprises a source electrode, a printablesemiconductor element, a gain electrode and a gate electrode. In thisconfiguration, the source electrode and gain electrode are both inelectrical contact with, and separated by, the printable semiconductorelement, and the gate electrode is separated from the printablesemiconductor element by the dielectric. The printable semiconductorelement may comprise a unitary crystalline inorganic semiconductorstructure having a thickness greater than or equal to about 50nanometers, preferably for some applications greater than or equal to100 nanometers and ever more preferably for some applications greaterthan or equal to 200 nanometers. The present invention also includes atransistor having a plurality of printable semiconductor elements incontact with source and drain electrodes. Use of a plurality ofprintable semiconductor elements in a single transistor may bebeneficial in some applications because it may reduce the overallpositional accuracy tolerances for various device components such as,source, drain and gate electrodes and dielectrics in field effecttransistors. The present invention also includes embodiments wherein theprintable semiconductor element is a stretchable semiconductor element.Use of one or more stretchable semiconductor elements in transistors ofthe present invention is beneficial because it provides good deviceperformance and mechanical ruggedness in flexed, stretched or deformeddevice orientations.

In another embodiment, the present invention provides a high performancetransistor supported by and/or in physical contact with a plasticsubstrate, such as a polyimide, polycarbonate or Mylar substrate.Transistors of this embodiment of the present invention may have aprintable semiconductor element comprising a single crystallineinorganic semiconductor structure, such as a silicon or germanium. Suchdevice configurations exhibit good device performance characteristics,such as field effect mobilities, threshold voltages, switchingfrequencies and on-off ratios. In an exemplary embodiment, a thin filmtransistor on a plastic substrate has a device field effect mobilitycomparable to the device field effect mobility of a transistor having asemiconductor element comprising a crystalline semiconductor fabricatedby conventional high temperature processing methods, for example adevice field effect mobility greater than or equal to 300 cm² V⁻¹ s⁻¹,more preferably greater than or equal to 800 cm² V⁻¹ s⁻¹. In anotherembodiment, the present invention provides a Si-MOS transistor having asingle crystalline silicon printable semiconductor element capable ofhigh frequency operation, such as operation at frequencies up to about280 MHz.

In another embodiment, the present invention provides complementarymetal-oxide semiconductor circuits comprising printable semiconductorelements. For example, printable semiconducting elements having lightlyN (or P) type doped area between two highly P (or N) type doped area areused to form CMOS circuits. This capability is particularly interestingfor applications requiring low power consumptions, as CMOS technologyhas a much smaller power dissipation over NMOS technology. Moreover, theCMOS technology has no static power dissipation, so this technology isparticularly well suited for battery operated electronic systems.Finally, circuits design using CMOS technology are usually more compactthan any over semiconductor technology, so more devices per surface areacan be integrated.

In one embodiment, the dielectric and the semiconductor components of atransistor of this aspect of the present invention may comprise aunitary composite printable semiconductor element. Alternatively, thedielectric, gate electrode and semiconductor element of a transistor ofthis aspect of the present invention may comprise a unitary compositeprintable semiconductor element. Use of a composite printablesemiconductor element having integrated semiconductor and insulatorstructures is preferred for some applications because it may providevery high quality dielectric-semiconductor interfaces in thin filmtransistors which exhibit very low leakage. In addition, use of acomposite printable semiconductor element having integratedsemiconductor and insulator structures also provides for efficientassembly of device components without the need for spin casting stepsfor integrating a dielectric layer in a thin film transistor.

In another embodiment, the present invention provides stretchablesemiconductor elements capable of withstanding significant strainwithout fracturing. Stretchable semiconductor conductor elements of thepresent invention may exhibit good electronic performance even whenundergoing significant strain, such as strain greater or equal about0.5%, preferably 1% and more preferably 2%. Stretchable semiconductorelements of the present invention preferred for some applications arealso flexible, and thus are capable of significant elongation, flexing,bending or deformation along one or more axes. Stretchablesemiconductors that are flexible may also exhibit good electronicperformance when in flexed, expanded, contracted, bent and/or deformedstates. Stretchable and flexible semiconductor elements of the presentinvention may be printable, and may comprise composite semiconductorelements having a semiconductor structure operationally connected toother device components, such as dielectrics, electrodes and othersemiconductors. The present invention includes a wide range ofelectronic and/or optoelectronic devices having stretchable and/orflexible semiconductor elements, such as transistors, diodes, LEDS,OLEDS, laser, micro- and nano-electromechanical devices.

A stretchable semiconductor element of the present invention comprises aflexible substrate having a supporting surface and a printablesemiconductor structure having a curved internal surface. In thisembodiment, at least a portion of the curved internal surface of thesemiconductor structure is bonded to the supporting surface of theflexible substrate. Exemplary semiconductor structures having curvedinternal surfaces useful in present invention comprise bentsemiconductor structures. In the context of this description, a “bentsemiconductor structure” refers to a semiconductor structure having acurved conformation resulting from the application of a force. Bentsemiconductor structures may have one or more folded regions. Bentsemiconductor structures may be present in a coiled conformation or in awrinkled conformation. Semiconductor structures having curved internalsurfaces, such as bent semiconductor structures, may be bonded to aflexible substrate in a conformation that is under strain, such as astrain less than about 30%, a strain less than about 10% or a strainless than 1%.

Curved internal surfaces of stretchable semiconductors of the presentinvention may have any contour profile providing stretchability orflexibility including, but not limited to, contour profilescharacterized by at least one convex region, at least on concave regionor a combination of at least one convex region and at least one concaveregion. In an embodiment, the curved internal surface of a stretchableand/or flexible semiconductor element has a contour profilecharacterized by a substantially periodic wave or a substantiallyaperiodic wave. In the context of this description, periodic andaperiodic waves may be any two or three dimensional wave form includingbut not limited to, sine waves, square waves, Aries functions, Gaussianwaves, Lorentzian waves, or any combination of these. For example, anstretchable and flexible semiconductor element of the present inventioncomprises a bent semiconductor ribbon having a curved internal surfacewith a contour profile characterized by a substantially periodic waveextending along the length of the ribbon. Stretchable and flexiblesemiconductor elements of this embodiment may be expandable orcontractible along an axis extending along the length of the ribbon, andmay be bendable or deformable along one or more other axes.

The contour profile of semiconductor structures in this embodiment ofthe present invention may change when mechanical stressed or when forcesare applied to the semiconductor element. Therefore, the ability tochange contour profile of exemplary semiconductor structures may providefor their ability to expand, contract, flex, deform and/or bend withoutexperiencing significant mechanical damage, fracture or a substantialreduction in electrical performance. The curved internal surface of thesemiconductor structure may be continuously bonded to the supportingsurface (i.e. bound at substantially all points along the curvedinternal surface). Alternatively, the curved internal surface of thesemiconductor structure may be discontinuously bonded to the supportingsurface, wherein the curved internal surface is bonded to the supportingsurface at selected points along the curved internal surface.

The present invention also includes stretchable electronic devicesand/or device components comprising a combination of a printablesemiconductor structure and additional integrated device components,such as electrical contacts, electrodes, conducting layers, dielectriclayers, and additional semiconductor layers (e.g. doped layers, P-Njunctions etc.), all of which having curved internal surfaces that aresupported by supporting surfaces of a flexible substrate. The curvedinternal surface configurations of the additional integrated devicecomponents enable then to exhibit good electronic performance even whenundergoing significant strain, such as maintaining electricalconductivity or insulation with a semiconductor element while in astretched or bent configuration. Additional integrated device componentsin this aspect of the present invention may have a bent configuration,such as a coiled or wrinkled configuration, as described above, and maybe fabricated using techniques similar to those used to fabricatestretchable semiconductor elements. In one embodiment, for example,stretchable device components, including a stretchable semiconductorelement, are fabricated independently and then interconnected.Alternatively, the semiconductor containing device may be fabricated ina planar configuration, and the resulting planar device is subsequentlyprocessed to provide curved internal surfaces to all or some of thedevice components.

Printable semiconductor elements of the present invention may compriseheterogeneous semiconductor elements exhibiting enhanced properties,such as enhanced mechanical, electrical, magnetic and/or opticalproperties, useful in a variety of device environments andconfigurations. Heterogeneous semiconductor elements are multicomponentstructures comprising a semiconductor in combination with one or moreadditives. In this context of this description, additives compriseelements, molecules and complexes, aggregates and particles thereof,that are different from the semiconductor in which they are combined,such as additives having a different chemical compositions and/orphysical states (e.g. crystalline, semicrystalline or amorphous states).Useful additives in this aspect of the invention include othersemiconductor materials, N-type and P-type dopants such as arsenic,boron, and antimony, structural enhancers, dielectric materials andconducting materials. Heterogeneous semiconductor elements of thepresent invention include structures having spatial homogeneouscompositions, such as uniformly doped semiconductor structures, andinclude structures having spatial inhomogeneous compositions, such assemiconductor structures having dopants with concentrations that varyspatially in one, two or three dimensions (i.e. a spatiallyinhomogeneous dopant distribution in the semiconductor element).

In another aspect, heterogeneous semiconductor elements comprisesemiconductor structures having additional integrated functional devicecomponents, such as dielectric layers, electrodes, electrical contacts,doped contact layers, P-N junctions, additional semiconductor layers,and integrated multilayer stacks for charge confinement. Additionalintegrated functional device components of this aspect of the presentinvention include both semiconductor-containing structures andnon-semiconductor-containing structures. In one embodiment,heterogeneous semiconductor elements comprise functional devices, suchas transistors, diodes or solar cells, or multielement functional devicecomponents that are capable of being effectively patterned, assembledand/or interconnected on a substrate material.

Use of printable heterogeneous semiconductor elements provides certainadvantages in the fabrication methods of the present invention. First,the “top down” processing approach of the present methods allowsvirtually any type of semiconductor processing, such as spatiallycontrolled doping, to be carried out in fabrication steps separate fromsubsequent fabrication steps of (i) defining the spatial dimensions ofthe semiconductor elements and (ii) assembling the semiconductorelements on substrates and into functional devices. Separation ofsemiconductor processing from devices and device component assembly andinterconnection in the present methods allows processing ofsemiconductor materials to be carried out under a range of conditionsuseful for generating very high quality semiconductor containingmaterials, including single crystal semiconductors having doped regionswith well defined concentrations and spatial dopant distributions, andintegrated, semiconductor multilayer stacks exhibiting high purities.For example, separation of semiconductor processing and device componentassembly, allows for semiconductor processing at high temperatures andunder conditions of highly controlled levels of impurities. Second, useof heterogeneous semiconductor elements comprising a plurality ofintegrated device components and/or functional devices allows forefficient high throughput printing of functional devices and arraysthereof in a commercially beneficial manner. For example, devicefabrication methods of the present invention using heterogeneoussemiconductor elements comprising a plurality of interconnected devicecomponents reduces the net number of fabrication steps and/or lowers thecosts involved in manufacturing some devices.

In another aspect, the present invention provides methods of assembling,positioning, organizing, transferring, patterning and/or integratingprintable semiconductor elements onto or into substrates via a range ofprinting methods, including dry transfer contact printing or solutionprinting techniques. Printing methods of the present invention arecapable of integrating one or more semiconductor elements onto or into asubstrate in a manner which does not substantially affect theirelectrical properties and/or mechanical characteristics. In addition,printing methods of the present invention are capable of assemblingsemiconductor elements onto or into selected regions of a substrate andin selected spatial orientations. Further, printing methods of thepresent invention are capable of integrating semiconductor elements andother device components into and/or onto a substrate in a mannerproviding high performing electronic and optoelectronic devices byestablishing good conductivity between selected device components, goodinsulation between selected device components and/or good spatialalignment and relative positioning between device components.

In one embodiment of the present invention, semiconductor elements areassembled onto a substrate surface by dry transfer contact printingmethods, such as soft lithographic microtransfer or nanotransfermethods. In one method, one or more printable semiconductor elements arecontacted with a conformable transfer device having one or more contactsurface(s). Contact established between the contact surface(s) and theprintable semiconductor element(s) binds or associates the semiconductorelement(s) to the contact surface(s). Optionally, conformal contact isestablished between the contact surface(s) and the printablesemiconductor element(s) to facilitate binding or associate of theseelements. At least a portion of the semiconductor element(s) disposed onthe contact surface(s) is subsequently contacted with a receivingsurface of the substrate. Optionally, the conformable transfer devicealso establishes conformal contact between the contact surface(s) havingthe semiconductor element(s) disposed thereon and at least a portion ofthe receiving surface. Separation of the contact surface of theconformable transfer device and the semiconductor element(s) transfersthe semiconductor element(s) onto the receiving surface, therebyassembling the semiconductor element on the receiving surface of thesubstrate. In an embodiment preferred for device fabricationapplications, printable semiconductor elements are positioned and/orintegrated onto the substrate in selected regions and in selectedspatial orientations. Optionally, the transfer process is repeatedmultiple times to provide patterning on large areas of a receivingsurface of a substrate. In this embodiment, the transfer stamp havingprintable semiconductor elements is contacted with a different region ofthe receiving substrate for each successive patterning step. In thismanner very large areas of a receiving surface may be pattern withsemiconductor elements derived from a single mother wafer.

An advantage of the use of dry transfer contact printing methods in thepresent invention is that patterns of printable semiconductors elementsmay be transferred and assembled onto substrate surfaces in a mannerpreserving selected spatial orientations of semiconductor elements whichdefine the pattern. This aspect of the present invention is particularlybeneficial for applications wherein a plurality of printablesemiconductor elements are fabricated in well defined positions andrelative spatial orientations which directly correspond to a selecteddevice configuration or array of device configurations. Transferprinting methods of the present invention are capable of transferring,positioning and assembling printable semiconductor elements and/oprintable semiconductor containing functional devices including, but notlimited to, transistors, optical waveguides, microelectromechanicalsystems, nanoelectromechanical systems, laser diodes, or fully formedcircuits.

In another embodiment, the present invention provides selective transferand assembly methods wherein some, but not all, of the printablesemiconductors provided are transferred and assembled onto or into asubstrate. In this embodiment, the conformable transfer device iscapable of binding selectively to specific printable semiconductorelements provided. For example, the conformable transfer device may havea selected three dimensional relief pattern on its external surfacehaving recessed regions and relief features. In this embodiment,recessed regions and relief features may be positioned such that onlyselected printable semiconductor elements are contacted by one or morecontact surfaces provided by the relief pattern, and subsequentlytransferred and assembled onto the substrate surface. Alternatively, theconformable transfer device may have a contact surface or plurality ofcontact surfaces having a selected pattern of binding regions, such aschemically modified regions having hydroxyl groups extending from thecontact surface and/or regions having one or more adhesive surfacecoatings. In this embodiment, only those semiconductor elements that arecontacted with the binding regions on the contact surface(s) are boundto the transfer device, and subsequently transferred and assembled ontothe substrate surface. An advantage of selective transfer and assemblymethods of the present invention is that a first pattern of printablesemiconductor elements characterized by a first set of positions andspatial orientations may be used to generate a second pattern ofprintable semiconductor elements different from the first pattern andcharacterized by a second set of positions and spatial orientations,corresponding to a selected device configuration or array of deviceconfigurations.

An exemplary conformable transfer device of the present inventioncomprises a dry transfer stamp, such as an elastomeric transfer stamp orcomposite, multi-layer patterning device. Conformable transfer devicesuseful for the present invention include patterning devices comprising aplurality of polymer layers as described in U.S. patent application Ser.No. 11/115,954, entitled “Composite Patterning Devices for SoftLithography”, filed with the U.S. Patent and Trademark Office on Apr.27, 2005, which is hereby incorporated by reference in its entirety. Anexemplary patterning device useable in the methods of the presentinvention comprises a polymer layer having a low Young's Modulus, suchas a poly(dimethylsiloxane) (PDMS) layer, preferably for someapplications having a thickness selected from the range of about 1micron to about 100 microns. Use of a low modulus polymer layer isbeneficial because it provides transfer devices capable of establishinggood conformal contact with one or more printable semiconductorelements, particularly printable semiconductor elements having curved,rough, flat, smooth and/or contoured exposed surfaces, and capable ofestablishing good conformal contact with substrate surfaces having awide range of surface morphologies, such as curved, rough, flat, smoothand/or contoured substrate surfaces.

Optionally, transfer devices of the present invention may furthercomprise a second layer having an external surface opposite an internalsurface, and having a high Young's modulus, such as high modulus polymerlayer, ceramic layer, glass layer or metal layer. In this embodiment,the internal surface of the first polymer layer and the internal surfaceof the second high modulus layer are arranged such that a force appliedto the external surface of the second high modulus layer is transmittedto the first polymer layer. Use of a high modulus second polymer layer(or backing layer) in transfer devices of the present invention isbeneficial because it provides transfer devices having a net flexuralrigidity large enough to provide good binding, transfer and assemblycharacteristics. For example, use of a transfer device having a netflexural rigidity selected from the range of about 1×10⁻⁷ Nm to about1×10⁻⁵ Nm minimizes distortions of the positions of semiconductorelements and/or other structures bound to the contact surface(s) uponestablishing conformal contact with a substrate surface. Use of a highmodulus, rigid backing layer also is beneficial for preventingdegradation of the printable semiconductor elements during transfer, forexample by prevent cracking of the printable semiconductor layers. Thisattribute provides methods and devices of assembling printablesemiconductor elements exhibiting high placement accuracy and goodpattern fidelity. Transfer devices of the present invention may compriseadditional layers, including polymer layers, for providing easy handlingand maintenance, good thermal properties and for providing uniformdistribution of a force applied to the transfer device to the entirecontact surface(s), as taught in U.S. patent application Ser. No.11/115,954, entitled “Composite Patterning Devices for Soft Lithography,filed with the U.S. Patent and Trademark Office on Apr. 27, 2005 whichis incorporated by reference in its entirety herein.

In another approach, the principles of ‘soft adhesion’ are used to guidethe transfer. Here, the viscoelastic nature of the surface material onthe transfer element leads to a peel force (i.e. the force that can liftobjects from a surface) that depends on peel rate. At high peel rates,this force is large enough to remove objects from a substrate andtransfer them onto a transfer element, even when the static surfaceenergy of the transfer element is lower than that of the substrate. Atlow peel rates, this peel force is low. In some embodiments, bycontacting a transfer element that supports an array of objects againsta final substrate, and then peeling the element away slowly leads to thetransfer of these objects from the transfer element to the substrate.This approach of the present invention using controlled peeling ratescan be used in combination with the other transfer approaches describedherein.

Transfer devices of the present invention may have a single continuouscontact surface or a plurality of discontinuous contact surfaces. Thecontact surface(s) of transfer devices of the present invention may bedefined by a selected three-dimensional dimensional relief patternhaving recessed regions and relief features having selected physicaldimensions. Contact surfaces useful in the present invention may becapable of binding printable semiconductor elements by van der Waalsforces, covalent bonds, adhesive layers, chemically modified regionssuch as regions having hydroxyl groups disposed on their surfaces,dipole-dipole forces or combinations of these. Transfer devices of thepresent invention may have contact surfaces having any area.

A number of methods may be used to facilitate transfer of printablesemiconductor elements from a contact surface into or onto a substratesurface. In an exemplary embodiment, the difference in surface energy ofthe substrate surface and the contact surface promotes transfer to thesubstrate surface. For example, transfer may be effectively achievedfrom a contact surface comprised of a PDMS layer having a lower surfaceenergy to a substrate surface having a higher surface energy, such as apolyimide, polycarbonate or Mylar surface. In addition, a plasticsubstrate surface may be softened or partially melted by heating priorto and/or during contact with printable semiconductor elements to betransferred, thereby generating semiconductor elements embedded in thesubstrate. Allowing the substrate to cool and harden prior to separationof the contact surface from the semiconductor elements promotesefficient transfer. Alternatively, the substrate surface may have one ormore chemical modified regions exhibiting an enhanced affinity of thesubstrate for the semiconductor elements. For example, modified regionsmay be covered by one or more adhesive layers or may be modified suchthat they undergo efficient covalent bonding, attractive van der Waalsforces, dipole-dipole forces or combinations of these with semiconductorelements to promote efficient transfer and assembly. Alternatively, apartial polymerized polymer precursor may be contacted with thesemiconductor elements or other device components and, subsequentlypolymerized, resulting in formation of a substrate having semiconductorelements embedded therein.

In an exemplary embodiment, printable semiconductor elements arefabricated having top surfaces coated with a thin release layer, such asa layer of photoresist used as a photomask patterned onto a substrateduring the definition and fabrication of the printable semiconductorelements. The contact surface of a conformable transfer device isbrought into conformal contact with the coated surfaces of the printablesemiconductor element. The release layer facilitates bonding of theprintable semiconductor elements to the contact surface of the transferdevice. Surfaces of the printable semiconductor elements not coated withthe release layer are then contacted with a receiving surface of asubstrate. Next, the release layer is removed, for example by exposureto an appropriate solvent such as acetone, thereby separating theprintable semiconductor elements from conformable transfer device.Optionally, the receiving surface may be coated with one or moreadhesive layers to facilitate transfer of the printable semiconductorelements.

In another embodiment of the present invention, printable semiconductorelements are assembled onto a substrate surface by solution printing. Inthe context of this description, the term “solution printing” isintended to refer to processes whereby one or more structures, such asprintable semiconductor elements, are dispersed into a carrier medium,such as a carrier fluid or solvent, and delivered in a concerted mannerto selected regions of a substrate surface. In an exemplary solutionprinting method, delivery of structures to selected regions of asubstrate surface is achieved by methods that are independent of themorphology and/or physical characteristics of the substrate surfaceundergoing patterning. In another embodiment, printable semiconductorelements remain suspended in a solvent until the solvent evaporates, oruntil an applied force, such as an electrostatic force, magnetic forceor force provided by an acoustic wave, pulls the printable semiconductorelements out of solution and onto selected regions of the substrate.This functionality may be provided by selection of appropriate physicaldimensions and masses of the printable semiconductor elements necessaryto avoid premature sedimentation. In this way, the solution printingmethods of the present invention differ materially from some fluidicself assembly methods in which elements suspended in a carrier mediumfall out of solution due to the force of gravity and statistically fallinto recessed regions of a substrate.

A method of assembling a printable semiconductor element on a receivingsurface of a substrate of the present invention comprises the stepdispersing a printable semiconductor element in a carrier medium,thereby generating a suspension comprising the semiconductor element inthe carrier medium. The semiconductor element is delivered to thesubstrate by solution printing the suspension onto the receivingsurface, thereby assembling the semiconductor element onto saidreceiving surface. In this embodiment, solution printing may be providedby a number of techniques known in the art including, but not limitedto, ink jet printing, thermal transfer printing, and screen printing.Solution printing methods of the present invention may also employ selfassembly alignment techniques. In one embodiment, for example,alignment, positioning and registration of printable semiconductorelements having patterned hydrophobic and hydrophilic groups are alignedon a receiving surface having complimentary patterned hydrophobicregions (such as methyl terminated surface groups) and hydrophilicregions (such as carboxylic acid terminated surface groups). Solutionprinting methods of the present invention may also employ capillaryaction of drops containing dispersed printable semiconductor elements toachieve alignment, positioning and registration.

Optionally, a number of methods may be used in the present invention tocontrol the orientation, alignment and selective deposition ofsemiconductor elements and/or other device components on the substratesurface. These methods enable fabrication of complex integratedelectronic and optoelectronic devices comprising a plurality ofinterconnected device components having precisely specified relativepositions and spatial orientations. For example, electrostatic forces,acoustic waves and/or magnetostatic forces may be employed to facilitatepositioning semiconductor elements and other device components inspecific locations and in selected spatial orientations on a substratesurface. Alternatively, the properties and/or composition of thesubstrate surface itself may be modified in selected regions to achieveaccurate placement of semiconductor elements and other devicecomponents. For example, selected regions of the substrate surface maybe chemically modified such that they exhibit a selective affinity forsemiconductor elements. In addition, the electrical properties of thesubstrate surface may be modified, for example by formation of potentialholes in specific surface regions, to facilitate selective integration,orientation and alignment of printable semiconductor elements and otherdevice components.

Printing methods of the present invention have a number of advantagesimportant for the fabrication of high performance electronic and/oroptoelectronic devices. First, printing methods of the present inventionare capable of transferring and assembling inorganic single crystallinesemiconductor structures without exposing these structures to mechanicalstrain large enough to induce significant damage or degradation, such asdamage from cracking. Second, printing methods of the present inventionare capable of positioning one or more semiconductor elements onselected regions of a substrate surface in selected orientations withgood placement accuracy (i.e. good spatial registration with respect toa selected region of a receiving surface), preferably with spatialdeviations from absolutely correct orientations and locations on asubstrate less than or equal to 5 microns. Third, printing methods ofthe present invention are capable of generating patterns comprising aplurality of semiconductor elements, other device elements, integratedfunction devices or any combination of these having good fidelity withrespect to a selected spatial configuration, such as a spatialconfiguration corresponding to a functional device or array of devices.Fourth, printing methods of the present invention may be carried out atrelatively low temperatures (i.e. temperatures less than about 400Celsius), and therefore are compatible with a wide range of substrates,particularly plastic substrates. Finally, printing methods of thepresent invention provide a low cost means of fabricating highperformance electronic and/or optoelectronic devices, and do not requireclean room conditions.

The present compositions and related methods of assembling, positioning,organizing, transferring, patterning and/or integrating printablesemiconductor elements onto or into substrates may be used to fabricatevirtually any structure comprising one or more semiconductor elements.These methods are particularly useful for fabricating complex integratedelectronic or optoelectronic devices or device arrays, such as arrays ofdiodes, light emitting diodes, solar cells, transistors (FET andbipolar), and thin film transistors. The present compositions andrelated methods are also useful for fabrication of systems levelintegrated electrical circuits, such as complementary logic circuits,wherein printable semiconductor elements are printed onto a substrate inwell defined spatial orientations and interconnected to form a desiredcircuit design. In one embodiment of this aspect of the invention,printable N-type and P-type heterogeneous semiconductor elements withselected dopant concentrations and dopant spatial distributions areassembled and interconnected to fabricate complex integrated circuits.In another embodiment, a plurality of printable semiconductor elementscomprising different semiconductor materials are printed onto the samesubstrate and interconnected to fabricate complex integrated circuits.

The assembly methods of the present invention, however, are not limitedto semiconductors. Rather these methods are broadly compatible with awide range of materials that are not semiconductors. Materials that maybe transferred and/or assembled by the methods of the present inventioninclude, but are not limited to, insulating materials such as SiO₂,connecting materials such as conductors, optical elements such as activeoptical materials, passive optical materials and fiber optic elements,materials for sensing applications and magnetic materials. Accordingly,the present methods, devices and device components may be used tofabricate a wide range of microsized and/or nanosized structures andassemblies of structures, such as microfluidic devices and structures,NEMS devices and arrays of NEMS devices and MEMS devices and arrays ofMEMS devices. Particularly, the transfer and assembly methods of thepresent invention may be used to generate complex three-dimensionalstructures, such as integrated circuits, by sequential overlay of aplurality of printing levels.

The present compositions, and related fabrication, assembly andinterconnection methods are useful for fabricating devices, particularlysemiconductor based devices, on large areas of a wide range ofsubstrates. A benefit of the present methods is that they are compatiblewith device assembly at temperatures that are compatible with mostflexible substrates, including polymer materials such as thermoplasticmaterials, thermoset materials, reinforced polymer materials andcomposite polymer materials. However, the present methods are equallyapplicable to device fabrication on rigid and/or brittle substratesincluding ceramic materials, glasses, dielectric materials, conductors,metals and semiconductor materials. The applicability of these methodsto device fabrication on brittle materials arises from the very lowforce imparted to substrates using the printing methods of the presentinvention. The present compositions and fabrication methods are alsocompatible with device fabrication on more unusual substrate materials,such as paper, wood and rubber, and contoured substrates, includingcurved substrates, curved rigid substrates, concave substrates, andconvex substrates. For example, the present methods are capable ofassembling and integrating printable semiconductor elements and otherdevice components (e.g. electrodes, dielectric layers, P-N junctionsetc.) on substrates, including rigid and flexible substrates, having aradius of curvature ranging from about 10 microns to about 10 meters.

In another aspect, the present fabrication methods are capable ofheterogeneous integration of printable semiconductor elements intofunctional substrates. For example, the printing methods of the presentinvention are capable of depositing and integrating printablesemiconductor elements into substrates having well defined semiconductorregions, conducting regions and/or insulating regions. An advantage ofthe present fabrication methods is that printable semiconductor elementscan be printed onto functional substrates, such as integrated circuitsor components of integrated circuits, in selected orientations andpositions with high placement accuracy, particularly in the context ofdry transfer contact printing methods of the present invention.

Printable semiconductor elements of the present invention may befabricated from a wide range of materials. Useful precursor materialsfor fabricating printable semiconductor elements include semiconductorwafer sources, including bulk semiconductor wafers such as singlecrystalline silicon wafers, polycrystalline silicon wafers, germaniumwafers; ultra thin semiconductor wafers such as ultra thin siliconwafers; doped semiconductor wafers such as P-type or N-type doped wafersand wafers with selected spatial distributions of dopants (semiconductoron insulator wafers such as silicon on insulator (e.g. Si—SiO₂, SiGe);and semiconductor on substrate wafers such as silicon on substratewafers and silicon on insulator. Further, printable semiconductorelements of the present invention may be fabricated from scrape orunused high quality or reprocessed semiconductor materials that are leftover from semiconductor device processing using conventional methods. Inaddition, printable semiconductor elements of the present invention maybe fabricated from a variety of nonwafer sources, such as a thin filmsof amorphous, polycrystalline and single crystal semiconductor materials(e.g. polycrystalline silicon, amorphous silicon, polycrystalline GaAsand amorphous GaAs) that is deposited on a sacrificial layer orsubstrate (e.g. SiN or SiO₂) and subsequently annealed.

The present invention also includes methods of making printablesemiconductor elements and flexible semiconductor elements. Thesemethods enable fabrication of printable semiconductor elements andflexible semiconductor elements from a wide range of precursormaterials, such as silicon on insulator wafers, single crystallinesilicon wafers, thin films of polycrystalline crystalline silicon, ultrathin silicon wafers and germanium wafers. In addition, these methods arecapable of generating printable semiconductor elements having a widerange of shapes and physical dimensions. Further, the present methodsenable low cost fabrication of large arrays/patterns of printablesemiconductor elements in well defined, relative spatial orientations.

In another aspect, the present invention provides a method forassembling a printable semiconductor element on a receiving surface of asubstrate comprising the steps of: (1) providing the printablesemiconductor element comprising a unitary inorganic semiconductorstructure; (2) contacting the printable semiconductor element with aconformable transfer device having a contact surface, wherein contactbetween the contact surface and the printable semiconductor elementbinds or associates the printable semiconductor element to the contactsurface, thereby forming the contact surface having the printablesemiconductor element disposed thereon; (3) contacting the printablesemiconductor element disposed on the contact surface with the receivingsurface of the substrate; and (4) separating the contact surface of theconformable transfer device and the printable semiconductor element,wherein the printable semiconductor element is transferred onto thereceiving surface, thereby assembling the printable semiconductorelement on the receiving surface of the substrate. In one embodiment,this method of the present invention further comprises the steps of: (1)providing additional printable semiconductor elements each of whichcomprising a unitary inorganic semiconductor structure; (2) contactingthe printable semiconductor elements with a conformable transfer devicehaving a contact surface, wherein contact between the contact surfaceand the printable semiconductor element binds or associates theprintable semiconductor elements to the contact surface and generatesthe contact surface having the printable semiconductor elements disposedthereon in relative orientations comprising a selected pattern of theprintable semiconductor elements; (3) contacting the printablesemiconductor elements disposed on the contact surface with thereceiving surface of the substrate; and (4) separating the contactsurface of the conformable transfer device and the printablesemiconductor elements, wherein the printable semiconductor elements aretransferred onto the receiving surface in the relative orientationscomprising the selected pattern.

In another aspect, the present invention provides a method forassembling a printable semiconductor element on a receiving surface of asubstrate comprising the steps of: (1) providing the printablesemiconductor element comprising a unitary inorganic semiconductorstructure, wherein the printable semiconductor element has at least onecross sectional dimension greater than or equal to about 500 nanometers;(2) dispersing the semiconductor element in a solvent, therebygenerating a suspension comprising the semiconductor element in thesolvent; and (3) delivering the semiconductor element to the substrateby solution printing the suspension onto the receiving surface therebyassembling the semiconductor element onto the receiving surface. In oneembodiment, this method of the present invention further comprises thesteps: (1) providing additional printable semiconductor elements,wherein each of the additional printable semiconductor elements has atleast one cross sectional dimension greater than or equal to about 500nanometers; (2) dispersing the semiconductor elements in the solvent,thereby generating a suspension comprising the semiconductor elements inthe solvent; and (3) delivering the semiconductor elements to thesubstrate by solution printing the suspension onto the receivingsurface, thereby assembling the semiconductor elements onto thereceiving surface.

In another aspect, the present invention provides a method offabricating a printable semiconductor element comprising the steps of:(1) providing a wafer having an external surface, the wafer comprising asemiconductor; (2) masking a selected region of the external surface byapplying a mask; (3) etching (optionally anisotropically etching) theexternal surface of the wafer, thereby generating a relief structure onthe wafer and at least one exposed surface of the wafer, wherein therelief structure has a masked side and one or more unmasked sides; (4)applying a mask to at least a portion of the unmasked sides of therelief structure; (5) at least partially etching the exposed surfaces ofthe wafer, thereby releasing a portion of the relief structure from thewafer and fabricating the printable semiconductor element. In thisembodiment, mask may be applied to the unmasked sides of the reliefstructure by angled deposition methods, such as sputtering or vapordeposition, or by flowing a portion of the mask on the external surfaceonto the unmasked sides

In another aspect, the present invention provides a method offabricating a printable semiconductor element comprising the steps of:(1) providing a wafer having an external surface, the wafer comprising asemiconductor; (2) masking selected regions of the external surface byapplying a first mask; (3) etching (optionally anisotropically etching)the external surface of the wafer, thereby generating a plurality ofrelief structures; (4) annealing the wafer, thereby generating anannealed external surface; (5) masking selected regions of the annealedexternal surface by applying a second mask; and (6) etching (optionallyanisotropically etching) the annealed external surface, therebygenerating the semiconductor element.

In another aspect, the present invention provides a method offabricating a printable semiconductor element comprising the steps of:(1) providing an ultra thin wafer having an external surface, the wafercomprising a semiconductor and having a selected thickness along an axisorthogonal to the external surface; (2) masking selected regions of theexternal surface by applying a mask; (3) etching (optionallyanisotropically etching) the external surface of the wafer, wherein thewafer is etched throughout the thickness along the axis orthogonal tothe external surface, thereby generating the printable semiconductorelement.

In another embodiment, the present invention provides a method formaking a flexible semiconductor element comprising the steps of: (1)providing a printable semiconductor structure having an internalsurface; (2) providing a prestrained elastic substrate in an expandedstate, wherein the elastic substrate has an external surface; and (3)bonding the internal surface of the printable semiconductor structure tothe external surface of the prestrained elastic substrate in an expandedstate; and allowing the elastic substrate to relax at least partially toa relaxed state, wherein relaxation of the elastic substrate bends theinternal surface of the printable semiconductor structure therebygenerating a semiconductor element having a curved internal surface. Inan exemplary embodiment, the prestrained elastic substrate is expandedalong a first axis, a second axis orthogonal to the first axis or both.The prestrained elastic substrate in an expanded state may be formed bybending the elastic substrate or rolling the elastic substrate.Optionally, the method of this aspect of the present invention mayfurther comprise the step of transferring the semiconductor having acurved internal surface to a receiving substrate that is flexible.

In another embodiment, the present invention provides a method forfabricating a printable semiconductor element connected to a motherwafer via one or more alignment maintaining elements comprising thesteps of: (1) providing the mother wafer having an external surface, thewafer comprising an inorganic semiconductor material; (2) masking aselected region of the external surface by applying a mask; (3) etchingthe external surface of the wafer, thereby generating a relief structureand at least one exposed surface of the wafer, wherein the reliefstructure has a masked side and one or more unmasked sides; (4) etchingthe exposed surfaces of the wafer; and (5) stopping etching of theexposed structure so that complete release of the relief structure isprevented, thereby fabricating the printable semiconductor elementconnected to a mother wafer via one or more alignment maintainingelements. In one embodiment of this method the printable semiconductorelement has a peanut shape with a first end and a second end, whereinthe alignment maintaining elements connect the first and second ends ofthe printable semiconductor element to the mother wafer. In anotherembodiment of this method the printable semiconductor element has aribbon shape with a first end and a second end, wherein the alignmentmaintaining elements connect the first and second ends of the printablesemiconductor element to the mother wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an exemplary method of the presentinvention for producing and assembling printable semiconductor elementscomprising ribbons of single crystalline silicon.

FIG. 2 provides a schematic diagram illustrating a selective drytransfer contact printing method for assembling printable semiconductorelement on a receiving surface of a substrate.

FIGS. 3A-3C, are schematic diagrams showing devices, deviceconfigurations and device components useful in selective dry transfercontact printing methods of the present invention. FIG. 3D provides aphotograph of an array of photodiodes printed onto a spherical surfaceof a polycarbonate lens (FL 100 mm). FIG. 3E provides a scanningelectron micrograph of an array of photodiodes printed onto the curvedsurface of a spherical glass lens (FL 1000 mm). Contrast in the imageprovided in FIG. 3E is slightly enhanced to show p-doped regions. FIG.3F provides a plot of electric current (μA) verse bias potential (volts)illustrating the light response of the photodiodes pictured in FIG. 3E.

FIGS. 4A1 and 4A2 show a preferred shape of a printable semiconductorelement for assembly methods of the present invention using dry transfercontact printing. FIG. 4A1 provides a perspective view and FIG. 4A2provides a top plan view. FIGS. 4B1 and 4B2 show a preferred shape of aprintable semiconductor element for assembly methods of the presentinvention using dry transfer contact printing. FIG. 4B1 provides aperspective view and FIG. 4B2 provides a top plan view.

FIGS. 5A-5C presents optical and scanning electron micrographs of arange of printable semiconductor elements comprising microstrips ofsingle crystalline silicon having selected physical dimensions

FIG. 6 presents an image of transferred printable semiconductor elementscomprising single crystalline silicon microstrips on a PDMS coatedpolyimide sheet.

FIG. 7 presents an optical micrograph image of a thin film transistorhaving a printable semiconductor element.

FIG. 8 provides a plot showing current-voltage (IV) characteristics of adevice made on a pre-oxidized Si wafer.

FIG. 9 provides a plot showing transfer characteristics measured atV_(DS)=0.1 V of a device made on a Mylar sheet coated with ITO gate andpolymer dielectric.

FIGS. 10A-10H provide a schematic diagrams illustrating a method of thepresent invention for making an array of thin film transistors havingcomposite printable semiconductor elements.

FIGS. 11A-11D provide diagrams illustrating a method of the presentinvention for making a printable device comprising integrated gateelectrode, gate dielectric, semiconductor, source electrode and drainelectrode.

FIG. 12A, FIG. 12B and FIG. 12C provide an atomic force micrographsshowing a stretchable printable semiconductor element of the presentinvention.

FIG. 13A, FIG. 13B, FIG. 13C, FIG. 13D and FIG. 13E show atomic forcemicrographs providing an expanded view of a semiconductor structurehaving curved internal surface.

FIG. 14A, FIG. 14B and FIG. 14C show atomic force micrographs of anarray of stretchable printable semiconductor elements of the presentinvention.

FIG. 15 shows optical micrographs of stretchable printable semiconductorelements of the present invention.

FIG. 16A and FIG. 16B show atomic force micrographs of a stretchableprintable semiconductor element of the present invention having asemiconductor structure bonded to a flexible substrate having a threedimensional relief pattern on its supporting surface.

FIG. 17 shows a flow diagram illustrating an exemplary method of makinga stretchable semiconductor element of the present invention.

FIG. 18A shows an exemplary method of making printable semiconductorelements from a Si—Ge epi substrate.

FIG. 18B shows an exemplary method for fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate.

FIG. 18C shows another exemplary method of fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate.

FIG. 18D shows yet another exemplary method of fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate.

FIG. 18E shows an exemplary method of fabricating printablesemiconductor elements from an ultra thin silicon substrate.

FIG. 18F shows an exemplary method for making printable semiconductorelements from a thin film of polycrystalline silicon on a supportingsubstrate.

FIG. 18G shows an exemplary method for making printable semiconductorelements from a thin film of polycrystalline silicon on a SiO₂substrate.

FIGS. 18H(1) and 18H(2) illustrate a method for making singlecrystalline semiconductor thin films using printable semiconductorelements of the present invention.

FIG. 18I shows an exemplary method of fabricating printablesemiconductor elements comprising micro-wires from GaAs substrate.

FIG. 18J shows an alternative method for fabricating printablesemiconductor elements comprising single crystalline silicon ribbons.

FIG. 18K shows an alternative method for fabricating printablesemiconductor elements comprising single crystalline silicon ribbons.

FIG. 19A provides a schematic diagram illustrating the steps of anexemplary method of generating nanowire arrays of GaAs and FIG. 19Bprovides a schematic diagram illustrating transferring nanowire arraysof GaAs to a substrate, such as plastic substrate comprisingpoly(ethylene terephthalate) (PET) sheet coated with a thin layer ofcured polyurethane (PU).

FIG. 20A provides a scanning electron micrograph of free-standing GaAswires obtained from GaAs wafer patterned with isolated SiO₂ lines. FIGS.20B-E show scanning electron micrograph images of individual wiresobtained by etching the GaAs wafer patterned with 2 μm wide SiO₂ lines.FIG. 20F provides a plot showing the dependence of the average width, w_(wires), of the top surfaces of wires fabricated by the present methodson etching time.

FIGS. 21A-21G shows images of a variety of GaAs wire arrays printed onPDMS and PU/PET substrates.

FIGS. 22A-22C shows scanning electron micrograph images of an InP wirearray on PMDS and PU/PET substrates.

FIG. 23A provides a schematic diagram and image of an exemplary twoterminal diode device comprising GaAs wire arrays. FIG. 23B shows thecurrent-voltage (I-V) curves recorded for the two terminal diode deviceat different bend radii indicating that the two terminal diode devicecomprising GaAs wire arrays exhibited expected diode characteristics.FIG. 23C shows the current-voltage (I-V) curves measured for the twoterminal diode device after relaxation after bending at different bendradii.

FIG. 24 provides a schematic diagram illustrating an exemplary method ofthe present invention for solution printing printable semiconductorelements having handle elements comprising magnetic tags.

FIG. 25 provides several optical images demonstrating the use ofsolution printing methods of the present invention to generate wellorder arrays of microstructures having handle elements comprising thinnickel layers.

FIG. 26A illustrates the steps used to fabricate exemplary bendable thinfilm transistors devices of the present invention. FIG. 26B presents aschematic illustration of the bottom gate device configuration of thethin film transistor together with high and low magnification opticalimages of part of the device array.

FIG. 27A presents current voltage characteristics of a bendable thinfilm transistor of the present invention that shows an effective devicemobility of 140 cm²/Vs in the saturation regime and 260 cm²/Vs in thelinear regime, as evaluated by application of standard field effecttransistor models that ignore the effects of contacts. FIG. 27B presentstransfer characteristics of several devices, plotted on linear (leftaxis) and logarithmic (right axis) scales. FIG. 27C shows thedistribution of the linear effective mobilities of several bendable thinfilm transistors fabricate by the present methods.

FIG. 28A presents a high-resolution scanning electron micrograph ofsolution cast ribbons (left inset) illustrating the remarkableflexibility of the printable single crystal silicon semiconductorelements. The right inset in FIG. 28A shows a picture of theexperimental setup used to bend the bendable thin film transistorsevaluated in this study. FIG. 28B shows the small (˜<1%) linearvariation of the epoxy dielectric capacitance when subject to tensileand compressive strains (see top inset). The lower inset in FIG. 28Bpresents the variation of the saturation current of a device measuredfor a gate and drain bias voltages of both 4 V.

FIG. 29A presents a schematic representation of a fabrication processfor generating transistors comprising printable heterogeneoussemiconductor elements on a PET substrate. FIG. 29B shows optical imagesof several devices having heterogeneous printable semiconductor elementsfabricated using the present techniques.

FIG. 30A shows a plot of the normalized resistance, R_(total)W, _(as) asa function of L. for an arrangement of printable heterogeneoussemiconductor elements and contact pads used to characterize the contactresistances (see inset). FIG. 30B shows Time-of-Flight Secondary IonMass Spectroscopy (TOF-SIMS) measurements that show the use of patternedSOG as a diffusion barrier (See, Schematic in FIG. 29A) localizes thedopants to desired regions in the silicon. In the image shown in FIG.30B, the bright red color indicates high phosphorous concentration.

FIGS. 31A-31D shows measurements corresponding to transistors comprisingprintable contact doped silicon semiconductor elements on anepoxy/ITO/PET substrate. FIG. 31A provides typical current-voltagecharacteristics of a single crystal silicon transistor with dopedcontacts on a PET substrate, with L=7 microns and W=200 microns. Fromthe bottom to top, V_(G) varies from −2 V to 6 V. FIG. 31 B providestransfer curves (V_(d)=0.1 V) of devices with channel lengths, from topto bottom, of 97 microns, 72 microns, 47 microns, 22 microns, 7 microns,and 2 microns. The channel width in each case is 200 microns. FIG. 31Cshows the width-normalized resistance of devices in the ON state(R_(on)W) as a function of channel length L at different gate voltages.The solid lines represent linear fits. The scaling is consistent withcontacts that have negligible influence on device performance for thisrange of channel lengths. the inset in FIG. 31 C shows the sheetconductance [Δ(R_(on)W)/Δ L]⁻¹, determined from the reciprocal of theslopes of the linear fitting in FIG. 31C, as a function of gate voltage.FIG. 31D shows effective mobility, evaluated in the linear regime, as afunction of channel length for the devices with undoped (triangle) anddoped (square) contacts.

FIG. 32A shows the change of the effective device mobility, normalizedby the value in the unbent state, μ_(0eff), as a function of strain (orbending radius). FIG. 32B presents normalized effective mobilitiesμ_(eff)/μ_(0eff) after several hundred bending cycles (to a radius of9.2 mm) that cause compressive strain at the device to vary between 0and 0.98%.

FIG. 33 shows an example of a composite semiconductor structurefabricated using a heterogeneous integration method of the presentinvention comprising gallium nitride microstructures direct-bonded ontoa silicon wafer (1 0 0).

FIG. 34A provides a process flow diagram schematically illustratingprocessing steps in a fabrication pathway for making a solar cellcomprising a printable P-N junction. 34B shows a schematic diagram of asolar cell device configuration generated by the fabrication pathwayillustrated in FIG. 34A. FIG. 34C shows the photodiode response observedupon illumination of a solar cell device having the configuration shownin FIG. 34B.

FIG. 35A provides a process flow diagram schematically illustratingprocessing steps in an alternative fabrication pathway for making asolar cell comprising printable P and N doped semiconductor layers. FIG.35B shows a schematic diagram of a solar cell device generated using thefabrication pathway illustrated in FIG. 35A. FIG. 35C shows a SEM imageof a top view of the solar cell schematically depicted in FIG. 35B. FIG.35D provides a plot of current verse bias demonstrating the photodioderesponse of the solar cell shown in FIG. 35C. FIG. 35E shows plots ofcurrent verse bias corresponding to several different illuminationintensities demonstrating the photodiode response of the solar cellshown in FIG. 35C.

FIG. 36A shows a process flow diagram illustrating an exemplary methodof making an array of stretchable thin film transistors. FIG. 36B showsprovides optical micrographs of an array of stretchable thin filmtransistors in relaxed and stretched configurations.

FIG. 37A provides a schematic diagram showing a processing method of thepresent invention (Method I) for patterning μs-Si elements onto aplastic substrate. FIG. 37B provides a schematic diagram illustrating analternative processing method of the present invention (Method II) forpatterning μs-Si elements onto a plastic substrate.

FIG. 38A shows the design of the so-called peanut shaped μs-Si objectsused in methods of the present invention. Inset optical image in FIG.38A shows the optimized HF etching condition where the buried oxideunder the channel is removed while a sacrificial SiO2 portion remains.FIG. 38B shows an example of lost of this order when the Si objects areoveretched in HF solution. FIGS. 38C, 38D, 38E and 38F shows a series ofmicrographs that depicts the progression of each step of the μs-Sitransfer as effected using Method I.

FIGS. 39A and 39B provide optical images of the selective transfer ofthe μs-Si onto PU/PET sheet by 3600 PDMS stamp. FIG. 39C is an opticalmicrograph of a section of a Sylgard 184 coated PET substrate to whichthe μs-Si has been chemically bonded and subsequently transferred. Ahigher magnification image of the μs-Si transferred in this way is shownin FIG. 39D.

FIG. 40A illustrates an exemplary device geometry of a device fabricatedusing the peanut shaped μs-Si based on a transfer using Method I. FIG.40B provides I-V curves of μs-Si TFTs at a range of gate voltage(Vg=−2.5 V to 20 V). FIG. 40C shows the transfer characteristics,measured at a constant source-drain voltage (Vsd=1V), indicated theeffective mobility was 173 cm²/Vs. The inset in FIG. 40C shows anoptical micrograph of actual device of the present invention.

FIG. 41 provides a schematic process flow diagram depicting stepsinvolved in the process for fabricating μs-GaAs MESFETs on apoly(ethylene terephthalate) (PET) substrate. Anisotropic chemicaletching produces wires from a standard (100) GaAs wafer. A printingtechnique that uses an elastomeric stamp transfers these wires from thewafer to the plastic device substrate in a manner that preserves spatialorganization (i.e. ordered arrays). PR denotes photoresist.

FIG. 42A presents a schematic showing a cross section view of thegeometry of a GaAs wire based MESFET on a plastic substrate (PU/PET).The source/drain electrodes form ohmic contacts to the n-GaAs layer.FIG. 42B shows a representative image of two GaAs wire based MESFETs onplastic each of which uses an array of ten GaAs wires, fabricatedaccording to the process flow diagram of FIG. 41. FIG. 42C shows theimage of a 2 cm×2 cm PET sheet with hundreds of transistors, clearlydemonstrating its flexibility.

FIGS. 43A, 43B and 43C present results from a GaAs MESFET with a channellength of 50 μm, a gate length of 15 μm, similar to the one shown inFIG. 42B. FIG. 43A shows the current-voltage (between drain and sourceelectrodes) curves at gate voltages between 0.5 to −2.0 V with steps of0.5 V. FIG. 43B shows the transfer characteristics (i.e., I_(DS) vs.V_(GS)) of a GaAs MESFET of the present invention measured at differentV_(DS). FIG. 43C shows the transfer curve at V_(DS)=4 V, plotted as(I_(DS))^(1/2) vs. V_(GS), clearly showing a linear relationship asexpected for a MESFET.

FIGS. 44A and 44B show gate-modulated current-voltage characteristics ofa GaAs wire based MESFET on a flexible PET substrate (A) before bending;(B) after bending to a bend radius of 8.4 mm. FIG. 44C shows thegate-modulated current-voltage characteristics of the GaAs wire basedMESFET after relaxing the bent substrate to its flat, unbent state. FIG.44D shows the variation of I_(DS) at V_(DS)=4 V and V_(GS)=0 V in 3cycles in terms of bending (with different surface strains)/unbending,indicating that these MESFETs survive multiple bending cycles that causethe tensile strain at the device to vary between 0% and 1.2%, withoutsignificant change of their performance (<20%).

FIG. 45 and FIG. 45 (cont'd) provide a schematic diagram illustrating anexemplary device configuration of the present invention for a P typebottom gate thin film transistor on a plastic substrate.

FIG. 46 provides a schematic diagram illustrating an exemplary deviceconfiguration of the present invention for a complementary logic gate ona plastic substrate.

FIG. 47 provides a schematic diagram illustrating an exemplary deviceconfiguration of the present invention for a top gate thin filmtransistor on a plastic substrate.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, like numerals indicate like elements and thesame number appearing in more than one drawing refers to the sameelement. In addition, hereinafter, the following definitions apply:

“Printable” relates to materials, structures, device components and/orintegrated functional devices that are capable of transfer, assembly,patterning, organizing and/or integrating onto or into substrateswithout exposure of the substrate to high temperatures (i.e. attemperatures less than or equal to about 400 degrees Celsius). In oneembodiment of the present invention, printable materials, elements,device components and devices are capable of transfer, assembly,patterning, organizing and/or integrating onto or into substrates viasolution printing or dry transfer contact printing.

“Printable semiconductor elements” of the present invention comprisesemiconductor structures that are able to be assembled and/or integratedonto substrate surfaces, for example using by dry transfer contactprinting and/or solution printing methods. In one embodiment, printablesemiconductor elements of the present invention are unitary singlecrystalline, polycrystalline or microcrystalline inorganic semiconductorstructures. In this context of this description, a unitary structure isa monolithic element having features that are mechanically connected.Semiconductor elements of the present invention may be undoped or doped,may have a selected spatial distribution of dopants and may be dopedwith a plurality of different dopant materials, including P and N typedopants. The present invention includes microstructured printablesemiconductor elements having at least one cross sectional dimensiongreater than or equal to about 1 micron and nanostructured printablesemiconductor elements having at least one cross sectional dimensionless than or equal to about 1 micron. Printable semiconductor elementsuseful in many applications comprises elements derived from “top down”processing of high purity bulk materials, such as high puritycrystalline semiconductor wafers generated using conventional hightemperature processing techniques. In one embodiment, printablesemiconductor elements of the present invention comprise compositestructures having a semiconductor operational connected to at least oneadditional device component or structure, such as a conducting layer,dielectric layer, electrode, additional semiconductor structure or anycombination of these. In one embodiment, printable semiconductorelements of the present invention comprise stretchable semiconductorelements and/or heterogeneous semiconductor elements.

“Cross sectional dimension” refers to the dimensions of a cross sectionof device, device component or material. Cross sectional dimensionsinclude width, thickness, radius, and diameter. For example, printablesemiconductor elements having a ribbon shape are characterized by alength and two cross sectional dimensions; thickness and width. Forexample, printable semiconductor elements having a cylindrical shape arecharacterized by a length and the cross sectional dimension diameter(alternatively radius).

“Fill factor” refers to the percentage of the area between two elements,such as first and second electrodes, that is occupied by a material,element and/or device component. In one embodiment of the presentinvention, first and second electrodes are provided in electricalcontact with one or more printable semiconductor elements that provide afill factor between first and second electrodes greater than or equal to20%, preferably greater than or equal to 50% for some applications andmore preferably greater than or equal to 80% for some applications.

“Supported by a substrate” refers to a structure that is present atleast partially on a substrate surface or present at least partially onone or more intermediate structures positioned between the structure andthe substrate surface. The term “supported by a substrate” may alsorefer to structures partially or fully embedded in a substrate.

“Solution printing” is intended to refer to processes whereby one ormore structures, such as printable semiconductor elements, are dispersedinto a carrier medium and delivered in a concerted manner to selectedregions of a substrate surface. In an exemplary solution printingmethod, delivery of structures to selected regions of a substratesurface is achieved by methods that are independent of the morphologyand/or physical characteristics of the substrate surface undergoingpatterning. Solution printing methods useable in the present inventioninclude, but are not limited to, ink jet printing, thermal transferprinting, and capillary action printing.

“Substantially longitudinally oriented” refers to an orientation suchthat the longitudinal axes of a population of elements, such asprintable semiconductor elements, are oriented substantially parallel toa selected alignment axis. In the context of this definition,substantially parallel to a selected axis refers to an orientationwithin 10 degrees of an absolutely parallel orientation, more preferablywithin 5 degrees of an absolutely parallel orientation.

“Stretchable” refers to the ability of a material, structure, device ordevice component to be strained without undergoing fracture. In anexemplary embodiment, a stretchable material, structure, device ordevice component may undergo strain larger than about 0.5% withoutfracturing, preferably for some applications strain larger than about 1%without fracturing and more preferably for some applications strainlarger than about 3% without fracturing.

The terms “flexible” and “bendable” are used synonymously in the presentdescription and refer to the ability of a material, structure, device ordevice component to be deformed into a curved shape without undergoing atransformation that introduces significant strain, such as straincharacterizing the failure point of a material, structure, device ordevice component. In an exemplary embodiment, a flexible material,structure, device or device component may be deformed into a curvedshape without introducing strain larger than or equal to about 5%,preferably for some applications larger than or equal to about 1%, andmore preferably for some applications larger than or equal to about0.5%.

“Semiconductor” refers to any material that is a material that is aninsulator at a very low temperature, but which has a appreciableelectrical conductivity at a temperatures of about 300 Kelvin. In thepresent description, use of the term semiconductor is intended to beconsistent with use of this term in the art of microelectronics andelectrical devices. Semiconductors useful in the present invention maycomprise element semiconductors, such as silicon, germanium and diamond,and compound semiconductors, such as group IV compound semiconductorssuch as SiC and SiGe, group III-V semiconductors such as AlSb, AlAs,Aln, AlP, BN, GaSb, GaAs, GaN, GaP, InSb, InAs, InN, and InP, groupIII-V ternary semiconductors alloys such as Al_(x)Ga_(1-x)As, groupII-VI semiconductors such as CsSe, CdS, CdTe, ZnO, ZnSe, ZnS, and ZnTe,group I-VII semiconductors CuCl, group IV-VI semiconductors such as PbS,PbTe and SnS, layer semiconductors such as PbI₂, MoS₂ and GaSe, oxidesemiconductors such as CuO and Cu₂O. The term semiconductor includesintrinsic semiconductors and extrinsic semiconductors that are dopedwith one or more selected materials, including semiconductor havingp-type doping materials and n-type doping materials, to providebeneficial electrical properties useful for a given application ordevice. The term semiconductor includes composite materials comprising amixture of semiconductors and/or dopants. Specific semiconductormaterials useful for in some applications of the present inventioninclude, but are not limited to, Si, Ge, SiC, AIP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, InP, InAs, GaSb, InP, InAs, InSb, ZnO, ZnSe, ZnTe, CdS,CdSe, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, PbS, PbSe, PbTe, AlGaAs, AlInAs,AlInP, GaAsP, GaInAs, GaInP, AlGaAsSb, AlGaInP, and GaInAsP. Poroussilicon semiconductor materials are useful for applications of thepresent invention in the field of sensors and light emitting materials,such as light emitting diodes (LEDS) and solid state lasers. Impuritiesof semiconductor materials are atoms, elements, ions and/or moleculesother than the semiconductor material(s) themselves or any dopantsprovided to the semiconductor material. Impurities are undesirablematerials present in semiconductor materials which may negatively impactthe electrical properties of semiconductor materials, and include butare not limited to oxygen, carbon, and metals including heavy metals.Heavy metal impurities include, but are not limited to, the group ofelements between copper and lead on the periodic table, calcium, sodium,and all ions, compounds and/or complexes thereof. Gold is a specificheavy metal impurity which significantly degrades the electricalproperties of semiconductors.

“Plastic” refers to any synthetic or naturally occurring material orcombination of materials that can be molded or shaped, generally whenheated, and hardened into a desired shape. Exemplary plastics useful inthe devices and methods of the present invention include, but are notlimited to, polymers, resins and cellulose derivatives. In the presentdescription, the term plastic is intended to include composite plasticmaterials comprising one or more plastics with one or more additives,such as structural enhancers, fillers, fibers, plasticizers, stabilizersor additives which may provide desired chemical or physical properties.

“Dielectric” and “dielectric material” are used synonymously in thepresent description and refer to a substance that is highly resistant toflow of electric current. Useful dielectric materials include, but arenot limited to, SiO₂, Ta₂O₅, TiO₂, ZrO₂, Y₂O₃, SiN₄, STO, BST, PLZT,PMN, and PZT.

“Polymer” refers to a molecule comprising a plurality of repeatingchemical groups, typically referred to as monomers. Polymers are oftencharacterized by high molecular masses. Polymers useable in the presentinvention may be organic polymers or inorganic polymers and may be inamorphous, semi-amorphous, crystalline or partially crystalline states.Polymers may comprise monomers having the same chemical composition ormay comprise a plurality of monomers having different chemicalcompositions, such as a copolymer. Cross linked polymers having linkedmonomer chains are particularly useful for some applications of thepresent invention. Polymers useable in the methods, devices and devicecomponents of the present invention include, but are not limited to,plastics, elastomers, thermoplastic elastomers, elastoplastics,thermostats, thermoplastics and acrylates. Exemplary polymers include,but are not limited to, acetal polymers, biodegradable polymers,cellulosic polymers, fluoropolymers, nylons, polyacrylonitrile polymers,polyimide-imide polymers, polyimides, polyarylates, polybenzimidazole,polybutylene, polycarbonate, polyesters, polyetherimide, polyethylene,polyethylene copolymers and modified polyethylenes, polyketones,poly(methyl methacrylate, polymethylpentene, polyphenylene oxides andpolyphenylene sulfides, polyphthalamide, polypropylene, polyurethanes,styrenic resins, sulphone based resins, vinyl-based resins or anycombinations of these.

“Elastomer” refers to a polymeric material which can be stretched ordeformed and return to its original shape without substantial permanentdeformation. Elastomers commonly undergo substantially elasticdeformations. Exemplary elastomers useful in the present invention maycomprise, polymers, copolymers, composite materials or mixtures ofpolymers and copolymers. Elastomeric layer refers to a layer comprisingat least one elastomer. Elastomeric layers may also include dopants andother non-elastomeric materials. Elastomers useful in the presentinvention may include, but are not limited to, thermoplastic elastomers,styrenic materials, olefenic materials, polyolefin, polyurethanethermoplastic elastomers, polyamides, synthetic rubbers, PDMS,polybutadiene, polyisobutylene, poly(styrene-butadiene-styrene),polyurethanes, polychloroprene and silicones.

The term “electromagnetic radiation” refers to waves of electric andmagnetic fields. Electromagnetic radiation useful for the methods of thepresent invention includes, but is not limited to, gamma rays, X-rays,ultraviolet light, visible light, infrared light, microwaves, radiowaves or any combination of these.

“Good electronic performance” and “high performance” are usedsynonymously in the present description and refer to devices and devicecomponents have electronic characteristics, such as field effectmobilities, threshold voltages and on-off ratios, providing a desiredfunctionality, such as electronic signal switching and/or amplification.Exemplary printable semiconductor elements of the present inventionexhibiting good electronic performance may have intrinsic field effectmobilities greater than or equal 100 cm² V⁻¹ s⁻¹, preferably for someapplications greater than or equal to about 300 cm² V⁻¹ s⁻¹. Exemplarytransistors of the present invention exhibiting good electronicperformance may have device field effect mobilities great than or equalto about 100 cm² V⁻¹ s⁻¹, preferably for some applications greater thanor equal to about 300 cm² V⁻¹ s⁻¹, and more preferably for someapplications greater than or equal to about 800 cm² V⁻¹ s⁻¹. Exemplarytransistors of the present invention exhibiting good electronicperformance may have threshold voltages less than about 5 volts and/oron-off ratios greater than about 1×10⁴.

“Large area” refers to an area, such as the area of a receiving surfaceof a substrate used for device fabrication, greater than or equal toabout 36 inches squared.

“Device field effect mobility” refers to the field effect mobility of anelectrical device, such as a transistor, as computed using outputcurrent data corresponding to the electrical device.

Conformal contact” refers to contact established between surfaces,coated surfaces, and/or surfaces having materials deposited thereonwhich may be useful for transferring, assembling, organizing andintegrating structures (such as printable semiconductor elements) on asubstrate surface. In one aspect, conformal contact involves amacroscopic adaptation of one or more contact surfaces of a conformabletransfer device e to the overall shape of a substrate surface. Inanother aspect, conformal contact involves a microscopic adaptation ofone or more contact surfaces of a conformable transfer device to asubstrate surface leading to an intimate contact with out voids. Theterm conformal contact is intended to be consistent with use of thisterm in the art of soft lithography. Conformal contact may beestablished between one or more bare contact surfaces of a conformabletransfer device and a substrate surface. Alternatively, conformalcontact may be established between one or more coated contact surfaces,for example contact surfaces having a transfer material, printablesemiconductor element, device component, and/or device depositedthereon, of a conformable transfer device and a substrate surface.Alternatively, conformal contact may be established between one or morebare or coated contact surfaces of a conformable transfer device and asubstrate surface coated with a material such as a transfer material,solid photoresist layer, prepolymer layer, liquid, thin film or fluid.

“Placement accuracy” refers to the ability of a transfer method ordevice to transfer a printable element, such as a printablesemiconductor element, to a selected position, either relative to theposition of other device components, such as electrodes, or relative toa selected region of a receiving surface. “Good placement” accuracyrefers to methods and devices capable of transferring a printableelement to a selected position relative to another device or devicecomponent or relative to a selected region of a receiving surface withspatial deviations from the absolutely correct position less than orequal to 50 microns, more preferably less than or equal to 20 micronsfor some applications and even more preferably less than or equal to 5microns for some applications. The present invention provides devicescomprising at least one printable element transferred with goodplacement accuracy.

“Fidelity” refers to a measure of how well a selected pattern ofelements, such as a pattern of printable semiconductor elements, istransferred to a receiving surface of a substrate. Good fidelity refersto transfer of a selected pattern of elements wherein the relativepositions and orientations of individual elements are preserved duringtransfer, for example wherein spatial deviations of individual elementsfrom their positions in the selected pattern are less than or equal to500 nanometers, more preferably less than or equal to 100 nanometers.

“Young's modulus” is a mechanical property of a material, device orlayer which refers to the ratio of stress to strain for a givensubstance. Young's modulus may be provided by the expression;

$\begin{matrix}{{E = {\frac{({stress})}{({strain})} = \left( {\frac{L_{0}}{\Delta\; L} \times \frac{F}{A}} \right)}};} & ({II})\end{matrix}$wherein E is Young's modulus, L₀ is the equilibrium length, ΔL is thelength change under the applied stress, F is the force applied and A isthe area over which the force is applied. Young's modulus may also beexpressed in terms of Lame constants via the equation:

$\begin{matrix}{{E = \frac{\mu\left( {{3\lambda} + {2\;\mu}} \right)}{\lambda + \mu}};} & ({III})\end{matrix}$wherein λ and μ are Lame constants. High Young's modulus (or “highmodulus”) and low Young's modulus (or “low modulus”) are relativedescriptors of the magnitude of Young's modulus in a give material,layer or device. In the present invention, a High Young's modulus islarger than a low Young's modulus, preferably about 10 times larger forsome applications, more preferably about 100 times larger for otherapplications and even more preferably about 1000 times larger for yetother applications.

In the following description, numerous specific details of the devices,device components and methods of the present invention are set forth inorder to provide a thorough explanation of the precise nature of theinvention. It will be apparent, however, to those of skill in the artthat the invention can be practiced without these specific details.

This invention provides methods and devices for fabricating printablesemiconductor elements and assembling printable semiconductor elementsonto substrate surfaces. The present invention provides a variety ofsemiconductor elements that are printable including single crystallineinorganic semiconductors, composite semiconductor elements comprising asemiconductor structure operationally connected to one or more otherdevice components, and stretchable semiconductor elements. The methods,devices and device components of the present invention are capable ofgenerating high performance electronic and optoelectronic devices andarrays of devices, such as thin film transistors on flexible plasticsubstrates.

FIG. 1 schematically illustrates exemplary methods of the presentinvention for producing and assembling printable semiconductor elementscomprising ribbons of single crystalline silicon. The process begins byproviding a silicon-on-insulator (SOI) substrate 100 having a thinsingle crystalline silicon layer 105, a buried SiO₂ layer 107 and Sihandling layer 108. Optionally, the surface native oxide layer on thinsingle crystalline silicon layer 105, if present, may be removed, forexample by exposing the surface of the SOI substrate 100 to dilute (1%)HF. Upon adequately stripping the native oxide layer, selected regionsof external surface 110 of SOI substrate 100 are masked, thereby forminga pattern of mask elements 120, masked regions 125 and exposed surfaceregions 127 on external surface 110. In the embodiment shown in FIG. 1,external surface 110 is patterned with rectangular aluminum and goldsurface layers which provide mask elements 120 that are capable ofinhibiting etching of the masked regions 125 of external surface 110.Mask elements 120 may have any size and shape including, but not limitedto, square, rectangular, circular, elliptical, triangular shapes or anycombinations of these shapes. In an exemplary embodiment, patterns ofAl/Au layers providing mask elements having desired geometries arefabricated using microcontact printing, nanocontact printing techniques,or photolithography, and etching methods (TFA for Au; AL-11 premixedCyantec etchant for Al). Deposition of mask elements comprising thinmetal films may be provided by an electron beam evaporator, such as aTemescal BJD1800, for example by sequential deposition of Al (20 nm; 0.1nm/s) and then Au (100 nm; 1 nm/s).

External surface 110 of SOI substrate 100 is anisotropically etcheddownward. As shown in FIG. 1, although material is selectively removedfrom exposed surface regions 127, mask elements 120 prevent etching ofmasked regions 125, thereby generating a plurality of relief features140 comprising single crystalline silicon structures having slightlyangled side walls 141. In an exemplary embodiment wherein relieffeatures have side walls 141 having a thickness 147 of about 100nanometers, exposed surface regions 127 are exposed totetramethylammonium hydroxide (TMAH) for about 3.5 minutes. In thisembodiment, etching generates smooth sidewalls on relief features 140 ofsingle crystal silicon having Al/Au mask elements 120, preferably withdeviations from average surface positions of less than 10 nanometers.Relief features 140 may be lifted off of the substrate 100 when theunderlying SiO₂ layer 107 is partially or completely isotropicallyetched away, for example using concentrated (49%) HF. Liftoff of therelief features 140 generates printable semiconductor elements 150comprising discrete single crystalline silicon structures having onesurface covered by a mask element. Mask elements 120, Al/Au layers inthe present example, may be removed or may be integrated directly into afinal device structure, for example, as the source and drain electrodesin a thin film transistor. As shown in FIG. 1, the printablesemiconductor elements 150 may be assembled onto the receiving surfaceof substrate surface 160, such as a plastic substrate, by either drytransfer contact printing techniques (schematically shown by arrow 166)or by solution casting methods (schematically shown by arrow 165). Bothassembly methods may be carried out at room temperature in an ambientenvironment and, therefore, are compatible with a wide range ofsubstrates, including low cost, flexible plastic substrates.

Use of dry transfer contact printing methods to assemble printablesemiconductor elements has the benefit of taking advantage of the knownorientations and positions of the printable semiconductor elements justprior to their liftoff from the SOI substrate. In this case, proceduressimilar to those of soft lithographic transfer printing techniques areused to move the printable semiconductor elements from the SOI (afteretching away the SiO₂ but before lifting off the silicon) to desiredlocations on the device substrate. In particular, a conformableelastomeric transfer element picks up the objects from the SOI surfaceand transfers them to a desired substrate. Similarly, the printablesemiconductor elements can be directly transferred onto thin plasticsubstrates by Au cold welding using receptacle pads defined on thesurface of the target substrate.

In an exemplary method, at least a portion of printable semiconductorelements 150 are brought into conformal contact with the contact surface170 of a conformable transfer device 175, such as an elastomerictransfer stamp, polymer transfer device or composite polymer transferdevice, thereby bonding at least a portion of printable semiconductorelements 150 onto the contact surface 170. Printable semiconductorelements 150 disposed on the contact surface 170 of conformable transferdevice 175 are brought into contact with a receiving surface ofsubstrate 160, preferably in a manner establishing conformal contactbetween contact surface 170 and the receiving surface of substrate 160.Contact surface 170 is separated from printable semiconductor elements150 in contact with receiving surface of substrate 160, therebyassembling printable semiconductor elements 150 onto the receivingsurface. This embodiment of the present invention is capable ofgenerating a pattern on the receiving surface comprising printablesemiconductor elements 150 in well defined positions and spatialorientations. In the embodiment shown in FIG. 1, printable semiconductorelements 150 are operationally connected to gold pads 162 present on thereceiving surface of substrate 160.

FIG. 2 provides a schematic diagram illustrating a selective drytransfer contact printing method for assembling printable semiconductorelement on a receiving surface of a substrate. A plurality of printablesemiconductor elements 300 are fabricated on a mother substrate 305 in afirst pattern 310 of printable semiconductor elements 300 characterizedby well defined positions and spatial orientations. A conformabletransfer device 315 having a contact surface 320 with a plurality ofdiscrete binding regions 325 is brought into conformal contact with atleast a portion of printable semiconductor elements 300 on mothersubstrate 305. Binding regions 325 on contact surface 320 arecharacterized by an affinity for printable semiconductor elements 310,and may be chemically modified regions, such regions having hydroxylgroups extending from the surface of a PDMS layer, or regions coatedwith one or more adhesive layers. Conformal contact transfers at least aportion of printable semiconductor elements 310 which contact bindingregions 325 on contact surface 320. Printable semiconductor elements 310transferred to contact surface 320 are brought into contact withreceiving surface 330 of substrate 335, which may be a flexiblesubstrate such as a plastic substrate. Subsequent separation ofsemiconductor elements 310 and contact surface 320 results in assemblyof the semiconductor elements 310 on receiving surface 330 of substrate335, thereby generating a second pattern 340 of printable semiconductorelements characterized by well defined positions and spatialorientations different from the first pattern of printable semiconductorelements 340. As shown in FIG. 2, the printable semiconductor elements340 that remain on mother substrate 305 are characterized by a thirdpattern 345 of printable semiconductor elements different from first andsecond patterns of printable semiconductor elements. Printablesemiconductor elements 340 comprising the third pattern 345 may besubsequently transferred to and/or assembled onto substrate 335 oranother substrate using the printing methods of the present invention,including selective dry transfer methods.

FIGS. 3A-C, are schematic diagrams showing devices, deviceconfigurations and device components useful in selective dry transfercontact printing methods of the present invention. FIG. 3A shows aplurality of printable semiconductor elements 300 on a mother substrate305, wherein selected printable semiconductor elements 300 have one ormore adhesive coatings 350. As shown in FIG. 3A, adhesive coatings 350are provided in a well defined pattern. FIG. 3B shows a conformabletransfer device 315 having a contact surface 320 with a plurality ofdiscrete binding regions 325 provided in a well defined pattern. FIG. 3Cshows a conformable transfer device 315 having a three dimensionalrelief pattern 355 comprising relief features 360 provided in a welldefined pattern. In the embodiment shown in FIG. 3C, relief pattern 355provides a plurality of contact surfaces 320 that may be optionallycoated with one or more adhesive layers. Patterns of adhesive coatings350, binding regions 325 and relief features 360 preferablycorresponding to relative positions and spatial orientations ofprintable semiconductor elements 300 in device configurations or devicearray configurations, such as thin film transistor array configurations.

Use of dry transfer printing methods are useful in the present inventionfor assembling, organizing and integrating printable semiconductorelements on substrates having a wide range of compositions and surfacemorphologies, including curved surfaces. To demonstrate this functionalcapability of the present methods and compositions, semiconductorelements comprising silicon photodiodes were printed directly (i.e. noadhesive) onto the curved surfaces of a variety of optical lenses usingdry transfer printing methods employing an elastomeric stamp. FIG. 3Dprovides a photograph of an array of photodiodes printed onto aspherical surface of a polycarbonate lens (FL 100 mm). FIG. 3E providesa scanning electron micrograph of an array of photodiodes printed ontothe curved surface of a spherical glass lens (FL 1000 mm). Contrast inthe image provided in FIG. 3E is slightly enhanced to show p-dopedregions. FIG. 3F provides a plot of electric current (μA) verse biaspotential (volts) illustrating the light response of the photodiodespictured in FIG. 3E.

FIGS. 4A1 and 4A2 show a preferred shape of a printable semiconductorelement for assembly methods of the present invention using dry transfercontact printing. FIG. 4A1 provides a perspective view and FIG. 4A2provides a top plan view. Printable semiconductor element comprises aribbon 500 extending along a central longitudinal axis 502 having afirst end 505, center region 510 and second end 515. As shown in FIG.4A, the width of ribbon 500 selectively varies along its length.Particularly, first and second ends 505 and 515 are wider than centerregion 510. In an exemplary method, ribbon 500 is formed by etchingmother substrate 520. In this embodiment, mother substrate isisotropically exposed to an enchant until ribbon 500 is only attached tomother substrate 520 by two alignment maintaining elements comprisingsacrificial layers 525 proximate to first and second ends 505 and 515.At this point in the fabrication process the etching process is stopped,and the ribbon 500 is brought into contact with and/or bonded to aconformable transfer device. Sacrificial layers 525 are broken andribbon 500 is released as the transfer device is moved away from mothersubstrate 520. This method may also be applied to dry transfer contactprinting of a plurality of printable semiconductor elements havingshapes as shown in FIG. 4. An advantage of this method of the presentinvention is that the orientations and relative positions of a pluralityof ribbons 500 on mother substrate 520 may be precisely preserved duringtransfer, assembly and integration steps. Exemplary ranges for thethickness of the sacrificial layers are ˜1 um down to ˜100 nm withribbons widths between ˜2 um and 100 um. Interestingly, the cleavage ofthe ribbons typically occurs at the extremity of the objects (very closeto the point/edge where the ribbons are attached to the mother wafer).Wide ribbons usually do not distort during the lift-off process are theyare bonded to the stamp.

FIGS. 4B1 and 4B2 show a preferred shape of a printable semiconductorelement for assembly methods of the present invention using dry transfercontact printing. FIG. 4B1 provides a perspective view and FIG. 4B2provides a top plan view. Printable semiconductor element comprisesribbons 527 extending along a parallel central longitudinal axes 528.Ribbons 527 are held in a selected position and orientation by alignmentmaintaining elements 530 which connect at least on end of the ribbonalong the central longitudinal axes 528 to mother substrate 529.alignment maintaining elements 530 are fabricated during patterning ofribbons 527 by not defining one or both ends of the ribbon along theircentral longitudinal axes. Alignment maintaining elements 530 are brokenand ribbons 527 are released upon contact with the ribbons with thecontact surface of a transfer device and subsequent movement away frommother substrate 520.

To achieve assembly by solution printing, at least a portion ofprintable semiconductor elements 150 are dispersed into a carriermedium, thereby generating a suspension 190 comprising semiconductorelement elements 150 the carrier medium. Printable semiconductorelements 150 are delivered to the substrate and assembled by solutionprinting the suspension onto the receiving surface of substrate 160.Solution printing may be provided by a number of techniques known in theart including, but not limited to, ink jet printing, thermal transferprinting and screen printing. In the embodiment shown in FIG. 1,printable semiconductor elements 150 are operationally connect to goldpads 162 present on the receiving surface of substrate 160.

FIG. 5A-C presents optical and scanning electron micrographs of a rangeof printable semiconductor elements 150 comprising microstrips of singlecrystalline silicon having selected physical dimensions. Printablesemiconductor elements are shown in ethanol suspensions and as cast ontosubstrates of various types. FIG. 5A shows an optical micrograph of asolution cast tangled mat of silicon rods (widths 2 microns; thickness 2microns; lengths ˜15 millimeters). The inset image shows printablesilicon strips (roughly 10 million of them) dispersed in a solution ofethanol. The low resolution SEM image in FIG. 5B illustrates themechanical flexibility range of some flat microstrips (thickness 340nanometers; widths 5 microns; lengths ˜15 millimeters) solution castedonto a bare silicon wafer. FIG. 5C presents a high resolution SEM imageof one of these objects. Note the extremely smooth sidewalls generatedby the anisotropic wet etching procedures.

Printable semiconductor elements in the form of wires, platelets anddisks may also be formed using the methods of the present invention. Byuse of large area soft lithographic techniques, it is possible, in asingle low cost processing sequence, to produce large numbers (i.e.billions) of printable semiconductor elements with lateral dimensionsdown to 50 nm and with nearly any geometry. Printable semiconductorelements having lateral dimension as small as 20 nanometer may also befabricated by the methods of present invention. For use in thin filmtransistors in flexible electronic systems, printable semiconductorelements comprising long (˜10 microns) and narrow (˜1 microns) strips ofsingle crystalline silicon are particularly useful.

FIG. 6 presents an image of transferred printable semiconductor elementscomprising single crystalline silicon microstrips on a PDMS coatedpolyimide sheet having a thickness of about 25 microns. The top insetpictures illustrate the intrinsic flexibility of this system. The bottominset shows a top view micrograph of printable silicon dense microstrips(25 microns wide, ˜2 microns spaced apart) cold welded on a thin Ti/Aucoated Mylar sheet. As shown in FIG. 6, the printable semiconductorelements comprising silicon microstrips are well aligned and transferredwith controlled orientation. No cracking of the printable semiconductorelements induced by assembly was observed upon careful examination usingscanning electron microscopy, even when the substrate was bentsignificantly. Similar results were obtained (without the need of anelastomeric layer) using a Au coated thin Mylar sheet as illustrated bythe bottom inset micrograph picture. A coverage density close to 100%can be achieved in this manner.

The present invention also provides composite printable semiconductorelements comprising a semiconductor structure operationally connect toone or more other device components, such as dielectric elements,conducting elements (i.e. electrodes) or additional semiconductorelements. An exemplary printable semiconductor elements of the presentinvention that is particularly useful for fabricating thin filmtransistors comprises an integrated semiconductor and dielectricelement. Such composite printable semiconductor elements providetransistors having high quality, leak free dielectrics and avoids theneed for separate spin casting steps for fabricating the dielectricelement in a thin film transistor. In addition, use of compositeprintable semiconductor elements enables efficient device fabrication onlarge substrate areas by low cost printing techniques.

The following references relate to self assembly techniques which may beused in methods of the present invention to transfer, assembly andinterconnect printable semiconductor elements via contact printingand/or solution printing techniques: (1) “Guided molecularself-assembly: a review of recent efforts”, Jiyun C Huie Smart Mater.Struct. (2003) 12, 264-271; (2) “Large-Scale Hierarchical Organizationof Nanowire Arrays for Integrated Nanosystems”, Whang, D.; Jin, S.; Wu,Y.; Lieber, C. M. Nano Lett. (2003) 3(9), 1255-1259; (3) “DirectedAssembly of One-Dimensional Nanostructures into Functional Networks”, YuHuang, Xiangfeng Duan, Qingqiao Wei, and Charles M. Lieber, Science(2001) 291, 630-633; and (4) “Electric-field assisted assembly andalignment of metallic nanowires”, Peter A. Smith et al., Appl. Phys.Lett. (2000) 77(9), 1399-1401.

All references cited in this application are hereby incorporated intheir entireties by reference herein to the extent that they are notinconsistent with the disclosure in this application. Some referencesprovided herein are incorporated by reference to provide detailsconcerning sources of starting materials, additional starting materials,additional reagents, additional methods of synthesis, additional methodsof analysis and additional uses of the invention. It will be apparent toone of ordinary skill in the art that methods, devices, device elements,materials, procedures and techniques other than those specificallydescribed herein can be applied to the practice of the invention asbroadly disclosed herein without resort to undue experimentation. Allart-known functional equivalents of methods, devices, device elements,materials, procedures and techniques specifically described herein areintended to be encompassed by this invention.

U.S. Patent Application Nos. 60/577,077, 60/601,061, 60/650,305,60/663,391 and 60/677,617 filed on Jun. 4, 2004, Aug. 11, 2004, Feb. 4,2005, Mar. 18, 2005, and May 4, 2005, respectively, are herebyincorporated by reference herein in their entireties to the extent notinconsistent with the disclosure of this application.

When a group of materials, compositions, components or compounds isdisclosed herein, it is understood that all individual members of thosegroups and all subgroups thereof are disclosed separately. When aMarkush group or other grouping is used herein, all individual membersof the group and all combinations and subcombinations possible of thegroup are intended to be individually included in the disclosure. Everyformulation or combination of components described or exemplified hereincan be used to practice the invention, unless otherwise stated. Whenevera range is given in the specification, for example, a temperature range,a time range, or a composition range, all intermediate ranges andsubranges, as well as all individual values included in the ranges givenare intended to be included in the disclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. In each instanceherein any of the terms “comprising”, “consisting essentially of” and“consisting of” may be replaced with either of the other two terms.

Example 1: Thin Film Transistor Having a Printable Semiconductor Element

The ability of printable semiconductor elements of the present inventionto provide semiconductor channels in thin film transistors was verifiedby experimental studies. Specifically, it is a goal of the presentinvention to provide thin film transistors capable of fabrication on aflexible plastic substrates by printing methods. Further, it is a goalof the present invention to provide high performance thin filmtransistors on plastic substrates having field effect mobilities, on-offratios and threshold voltages similar to or exceeding thin filmtransistors fabricated by convention high temperature processingmethods.

FIG. 7 presents an optical micrograph image of a thin film transistorhaving a printable semiconductor element. The illustrated transistor 531comprises source electrode 532, drain electrode 533, printablesemiconductor element 534, dielectric (not shown in the micrograph inFIG. 7) and gate electrode (also not shown in the micrograph in FIG. 7).The thin film transistor is supported by a substrate comprising of aMylar sheet coated with indium tin oxide (ITO, ˜100 nanometers thick) asa gate and a photocured epoxy as a gate dielectric (SU8-5; MicrochemCorp). The capacitance of the dielectric (2.85 nF/cm²) was evaluatedusing capacitor test structures formed near the device. This device usesa solution cast printable semiconductor element comprising a ˜5millimeter long, 20 micron width and 340 nanometer thick microstripfabricated from a p-doped SOI wafer (Soitec) with a 340 nanometer devicelayer thickness and resistivity of 14-22 ohm cm. A 25 nanometer thicklayer of SiO₂ was grown on top of the silicon by dry oxidation in ahorizontal quartz tube furnace. Source and drain electrodes of Al (20nanometer)/Au (180 nanometer) where defined by liftoff techniques. Thesemiconductor channel length is 50 microns and the width is 20 microns.

FIGS. 8 and 9 show electrical measurements collected from thin filmtransistors of the present invention having a printable semiconductorelement. The device operates similarly to a back gated SOI device with atop contact configuration. The semiconductor uses a width equal to a 20microns microstrip of single crystal silicon in a channel whose lengthis equal to 50 microns. The printable semiconductor element in this casewas patterned by solution casting methods. The source/drain contactswere defined by photolithography and lift off.

FIG. 8 provides a plot showing current-voltage (IV) characteristics of adevice made on a pre-oxidized Si wafer. FIG. 9 provides a plot showingtransfer characteristics measured at V_(DS)=0.1 V of a device made on aMylar sheet coated with ITO gate and polymer dielectric. The slope ofthis curve defines an effective device mobility (using the physicalwidth of the source and drain electrodes, which is equal to the width ofthe semiconductor element microstrip in this case) of 180 cm²/Vs. TheAl/Au metallization for the contacts to the printable semiconductorelement provides reasonably low resistance Schottky barrier contacts tothe silicon, as expected for an Al (work function of 4.2 eV)metallization on p-doped silicon. Aluminum is well known to diffuserapidly into silicon, but no special care was taken to avoid localizedaluminum-silicon interactions as no post metallization high temperatureannealing step was carried out. The on/off ratio of this device isslightly lower than 10³. Analysis of the transfer characteristic of FIG.9 indicates a linear field effect mobility of 180 cm² V⁻¹ s⁻¹ using aparallel plate model for the dielectric capacitance. This analysisignores the effects of contacts and processing induced changes in thethreshold voltage.

Even with perfect contacts, there are theoretical arguments to suggestthat transistors which incorporate very high aspect ratio (i.e. ultralarge length to width ratios) semiconducting elements in the channelregion (i.e. nanotubes or nanowires) will have responses that aredifferent than those of conventional devices. To avoid these effects, wechose printable semiconductor elements comprising microstrips havingwidths on the same order of magnitude with the transistor channellength. The properties (mobilities, normalized transconductance, on/offratio) observed here are ˜¾ to those of thin film transistors made onthe SOI substrate after etching of the Si but before liftoff. In thesemeasurements the buried SiO₂ oxide acts as the dielectric and thesilicon supporting substrate acts as the gate electrode. This resultdemonstrates that the processing steps used to produce the printablesemiconductor elements and to transfer it to the device substrate do notalter significantly the properties of the silicon or its surfaces thatresult from the initial patterning and silicon etching steps. It alsoindicates that the van der Waals interface with the SU8 dielectric iscapable of supporting good device properties.

A principle advantage of the fabrication method of the present exampleis that it separates the crystal growth and processing of the siliconfrom the plastic substrate and other components of the devices. Inaddition, the methods of processing printable semiconductor elements ofthe present invention are highly flexible in the processing sequencesand in the materials choices that are possible. For example, an SiO₂layer can be formed on one side of the silicon (by, for example, growinga thermal oxide before lifting off the Si elements or lifting the SOIburied oxide together with the Si device layer) to yield an integrateddielectric, in a strategy similar to that for the integratedsource/drain metallization demonstrated here. A dielectric introduced inthis manner may avoid the significant challenges that can be associatedwith leakage, hysteresis, doping, trapping, etc. in many solution castthin dielectrics on plastic substrates.

FIGS. 10 A-H provide a schematic diagram illustrating a method of thepresent invention for making an array of thin film transistors havingcomposite printable semiconductor elements. As shown in FIG. 10 A, gateelectrodes 547 are deposited on the surface 548 of a thin sheet of aflexible substrate, such as Kapton, Mylar or PET. Gate electrodes may bepatterned on the flexible substrate by any means known in the artincluding but not limited to photolithography, microtransfer printing,nanotransfer printing, soft lithography or combinations of these. Asshown in FIG. 10B, the method further comprises the step of fabricatinga plurality of composite printable semiconductor elements 550 comprisingsingle crystalline silicon structures 555 operationally connected to aSiO₂ dielectric element 560. As illustrated in FIG. 10B, compositeprintable semiconductor elements 550 have a ribbon shape extending aselected length 552 along a central longitudinal axis 551. Compositeprintable semiconductor element 550 has a selected thickness 553 and awidth that varies as a function of thickness.

As shown in FIG. 10C, the method further comprises the step ofassembling composite printable semiconductor elements 550 onto gateelectrodes 547 and substrate 548 via dry transfer contact printing orsolution printing. Composite printable semiconductor elements 550 areoriented such that SiO₂ dielectric elements 560 are in contact with gateelectrodes 547. As shown in FIG. 10D, the method further comprises thestep of spin coating a thin layer of positive photoresist 561 on thepatterned surface of substrate 548. Alternatively, the thin layer ofpositive photoresist 561 may be applied to the pattern surface ofsubstrate 548 using a roller. Regions of photoresist 561 not masked bygate electrodes 547 are exposed to a beam of electromagnetic radiationtransmitted through underside 562 of substrate 548. Use of an opticallytransmissive substrate 548 is preferred for this method of the presentinvention, particularly a substrate 548 that is at least partiallytransparent in ultraviolet and/or visible regions of the electromagneticspectrum. As shown in FIG. 10E, the method further comprises the step ofdeveloping the thin photoresist layer. As shown in this figure, theregions of thin photoresist layer 561 that are shadow masked by the gateelectrodes are undeveloped. As shown in FIG. 10F, the method furthercomprises the step of dry or wet etching the integrated SiO₂ dielectric,thereby opening contacts for source and drain electrodes. In theembodiment illustrated by FIG. 10F this is achieved by exposing thepatterned surface of substrate 548 to a CF₄ plasma. As shown in FIG.10G, the method further comprises the step of defining source and drainelectrodes by shadow mask evaporation. The alignment of semiconductorelements, source electrodes and drain electrodes does not need to bevery precise because the semiconductor channels will be defined in thenext fabrication step. As shown in FIG. 10H, the method furthercomprises the step of defining the semiconductor channel by lifting offthe positive resist, for example by exposure to a solvent such asacetone.

FIGS. 11A-D provide diagrams illustrating a method of the presentinvention for making a printable device comprising integrated gateelectrode, gate dielectric, semiconductor, source electrode and drainelectrode. As shown in FIG. 11A, a high quality gate dielectric is grownby thermal oxidation of the surface of a SOI wafer. Next, the gateelectrode material (such as metal or doped poly-silicon) is deposited.Selected regions of the top surface are subsequently masked using forexample a lithography process. In one embodiment, an array of identicalpatterns with controlled spacing will be defined in a single maskingstep. Printable semiconductor elements are then fabricated byanisotropically wet and/or dry etching. Preferentially, three differentselective etching processes are carried out sequentially to etch awaythe exposed areas of the gate electrode material, the gate dielectricand the top silicon layer.

A lithography process, as shown in FIG. 11B, is used to define thechannel of the transistors. In this process step, the exposed areas ofthe gate electrode material are etched away (dry or wet etching). Asshown in FIG. 11C, the photo-resist is then heated above its glasstransition, thereby initiating a reflow process. The reflowing distanceof the photoresist can be selected by carefully selecting an appropriatethickness of the photo-resist layer, the glass transition temperature ofthe photo-resist layer or the temperature and duration of the reflowprocess. The exposed areas of the gate dielectric are then etched usingan HF solution.

Next, a metallization process, as shown in FIG. 11D, is carried out,followed by lifting off the metal deposited onto the photoresist tocomplete the fabrication of a printable device. The source and drainelectrodes are self aligned with the gate, and the spacing betweensource and drain electrodes may be selected by the adjusting thedifferent parameters, such as temperature and duration, of the reflowprocess.

The printable device shown in FIG. 11D may be transferred and assembledonto a substrate, such as a plastic substrate, by the dry transfer orsolution printing methods of the present invention. The self alignedprocess illustrated in FIGS. 11A-D presents a simple way to integrateall the elements necessary for the realization of a printable device,such as a MOSFET device. A significant advantage of this fabricationmethod of the present invention is that all processes steps whichrequire temperatures incompatible with plastic substrates (e.g.requiring temperature >about 400 Celsius) may be carried on the SOIsubstrate prior to lifting off and transferring the device to thesubstrate. For example, additional processing steps such as doping ofthe source and drain contact areas, formation of silicide layers, andhigh temperature annealing of the device could be performed prior totransferring the elements onto a plastic substrate.

Example 2: Stretchable Printable Semiconductor Elements

The present invention provides stretchable printable semiconductorelements capable of providing good performance when stretched, flexed ordeformed. Further, stretchable printable semiconductor elements of thepresent invention may be adapted to a wide range of deviceconfigurations to provide fully flexible electronic and optoelectronicdevices.

FIG. 12 provides an atomic force micrograph showing a stretchableprintable semiconductor element of the present invention. Thestretchable printable semiconductor element 700 comprises a flexiblesubstrate 705 having a supporting surface 710 and a bent semiconductorstructure 715 having a curved internal surface 720. In this embodiment,at least a portion of the curved internal surface 720 of bentsemiconductor structure 715 is bonded to the supporting surface 710 ofthe flexible substrate 705. The curved internal surface 720 may bebonded supporting surface 710 at selected points along internal surface720 or at substantially all points along internal surface 720. Theexemplary semiconductor structure illustrated in FIG. 12 comprises abent ribbon of single crystalline silicon having a width equal to about100 microns and a thickness equal to about 100 nanometers. The flexiblesubstrate illustrated in FIG. 12 is a PDMS substrate having a thicknessof about 1 millimeter. Curved internal surface 720 has a contour profilecharacterized by a substantially periodic wave extending along thelength of the ribbon. As shown in FIG. 12, the amplitude of the wave isabout 500 nanometers and the peak spacing is approximately 20 microns.FIG. 13 shows an atomic force micrograph providing an expanded view of abent semiconductor structure 715 having curved internal surface 720.FIG. 14 shows an atomic force micrograph of an array of stretchableprintable semiconductor elements of the present invention. Analysis ofthe atomic force micrograph in FIG. 14 suggests that the bentsemiconductor structures are compressed by about 0.27%. FIG. 15 showsoptical micrographs of stretchable printable semiconductor elements ofthe present invention.

The contour profile of curved surface 720 allows the bent semiconductorstructure 715 to be expanded or contracted along deformation axis 730without undergoing substantial mechanical strain. This contour profilemay also allow the semiconductor structure to be bent, flexed ordeformed in directions other than along deformation axis 730 withoutsignificant mechanical damage or loss of performance induced by strain.Curved surfaces of semiconductor structures of the present invention mayhave any contour profile providing good mechanical properties, such asstretchability, flexibility and/or bendability, and/or good electronicperformance, such as exhibiting good field effect mobilities whenflexed, stretched or deformed. Exemplary contour profiles may becharacterized by a plurality of convex and/or concave regions, and by awide variety of wave forms including sine waves, Gaussian waves, Ariesfunctions, square waves, Lorentzian waves, periodic waves, aperiodicwaves or any combinations of these. Wave forms useable in the presentinvention may vary with respect to two or three physical dimensions.

FIG. 16 shows an atomic force micrograph of a stretchable printablesemiconductor element of the present invention having a bentsemiconductor structure 715 bonded to a flexible substrate 705 having athree dimensional relief pattern on its supporting surface 710. Thethree-dimensional relief pattern comprises recessed region 750 andrelief features 760. As shown in FIG. 16, bent semiconductor structure715 is bound to supporting surface 710 in recessed region 750 and onrelief features 760.

FIG. 17 shows a flow diagram illustrating an exemplary method of makinga stretchable semiconductor element of the present invention. In theexemplary method, a prestrained elastic substrate in an expanded stateis provided. Prestraining can be achieved by any means known in the artincluding, but not limited to, roll pressing and/or prebending theelastic substrate. An exemplary elastic substrate useable in this methodof the present invention is a PDMS substrate having a thickness equal toabout 1 millimeter. The elastic substrate may be prestrained byexpansion along a single axis or by expansion along a plurality of axes.As shown in FIG. 17, at least a portion of the internal surface of aprintable semiconductor structure is bonded to the external surface ofthe prestrained elastic substrate in an expanded state. Bonding may beachieved by covalent bonding between the internal surface of thesemiconductor surface, by van der Waals forces, by using adhesive or anycombinations of these. In an exemplary embodiment wherein the elasticsubstrate is PDMS, the supporting surface of the PDMS substrate ischemically modified such that is has a plurality of hydroxyl groupsextending from its surface to facilitate covalent bonding with a siliconsemiconductor structure. Referring back to FIG. 17, after binding theprestrained elastic substrate and semiconductor structure, the elasticsubstrate is allowed to relax at least partially to a relaxed state. Inthis embodiment, relaxation of the elastic substrate bends the internalsurface of said printable semiconductor structure, thereby generating asemiconductor element having a curved internal surface.

As shown in FIG. 17, the fabrication method may optionally include asecond transfer step wherein the semiconductor structure 715 having acurved internal surface 720 is transferred from the elastic substrate toanother substrate, preferably a flexible substrate. This second transferstep may be achieved by bringing an exposed surface of the semiconductorstructure 715 having a curved internal surface 720 in contact with areceiving surface of the other substrate that binds to the exposedsurface of the semiconductor structure 715. Bonding to the othersubstrate may be accomplished by any means in the are including covalentbonds, bonding via van der Waals forces and the use of adhesives.

Stretchable semiconductor elements of the present invention may beeffectively integrated into a large number functional devices and devicecomponents, such as transistors, diodes, lasers, MEMS, NEMS, LEDS andOLEDS. Stretchable semiconductor elements of the present invention havecertain advantages over conventional rigid inorganic semiconductors.First, stretchable semiconductor elements may be flexible, and thus,less susceptible to structural damage induced by flexing, bending and/ordeformation than conventional rigid inorganic semiconductors. Second, asa bent semiconductor structure may be in a slightly mechanicallystrained state to provide a curved internal surface, stretchablesemiconductor elements of the present invention may exhibit higherintrinsic field effect mobilities than conventional unstrained inorganicsemiconductors. Finally, stretchable semiconductor elements are likelyto provide good thermal properties because they are capable of expandingand contracting freely upon device temperature cycling.

Example 3: Methods of Making Printable Semiconductor Elements

The present invention provides methods of making printable semiconductorelements from a wide range of starting materials, including singlecrystalline wafers, silicon on substrate wafers, germanium wafers, thinfilms of polycrystalline silicon and ultra thin silicon wafers.Particularly, the present invention provides low cost methods of makinglarge numbers of printable semiconductors in selected orientations andrelative positions.

FIG. 18A shows an exemplary method for making printable semiconductorelements from a Si—Ge epi substrate. In this method, selective regionsof a Si epi layer are masked by depositing a mask material, such as athin films comprising a metal, SiO₂ or SiN. This masking step definesshape and some of the physical dimensions (e.g. length and width for aribbon) of the printable semiconductor elements to be fabricated. Theexposed Si surface of the Si—Ge epi substrate is anisotropically etchedby either dry or wet chemical etching methods. This generates relieffeatures of silicon, preferably having smooth side walls, that can beeffectively released from the Si—Ge epi substrate by lift offtechniques, for example using selective SiGe wet etching provided byNH₄OH:H₂O₂:H₂O 1:1:4 at 50° C. Optionally, source electrode, gainelectrode, gate electrode, dielectric element or any combinations ofthese may be integrated into the semiconductor element prior to liftoff. An advantage of this fabrication method is that the mothersubstrate can be cleaned and re-used.

FIG. 18B shows an exemplary method for fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate. In this method, a silicon wafer isfirst dry oxidized, for example in a quartz tube furnace at atemperature selected from the range of about 800 degrees Celsius toabout 1200 degrees Celsius. Next, a thin layer of gate material isdeposited on the oxidized surface of the silicon wafer. Exemplary gatematerials include metals or doped polycrystalline silicon. The thinlayer of gate material is selectively patterned with photoresist. Thispatterning step defines the shape and some of the physical dimensions(e.g. length and width for a ribbon) of the printable semiconductorelements to be fabricated. The thin layer of gate material anddielectric layer are anisotropically back etched, thereby generatingrelief features comprising a photoresist layer, gate material layer,dielectric layer and silicon layer and, preferably having smooth sidewalls. Next, the photoresist layer is reflowed, for example by annealingto a temperature selected from the range of about 100 degrees Celsius toabout 130 degrees Celsius. Reflowing the photoresist transfers a portionof the photoresist to the side walls of the relief features. As shown inFIG. 18B, the exposed Si surface is isotropically etch using wet or dryetching methods, thereby releasing the relief features and generatingcomposite printable semiconductor elements, preferably having smoothsurfaces. Isotropic etching of the silicon may be achieved using aHNO₃:NH₄F:H₂O 64:3:33 solution. Advantages of this fabrication methodare the relatively low cost of the silicon substrate starting materialsand the ability to reuse the mother substrate after planarization(ECMP).

FIG. 18C shows another exemplary method of fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate. In this method, the externalsurface of a bulk silicon substrate is selectively patterned withphotoresist. This patterning step defines the shape and some of thephysical dimensions (e.g. length and width for a ribbon) of theprintable semiconductor elements to be fabricated. The patternedsubstrate surface is anisotropically etched, preferably using dryetching methods such as reactive ion etching and inductive coupledplasma etching, thereby generating relief features, preferably relieffeatures having smooth side walls. At least a portion of the side wallsof the relief features are masked by depositing a masking material suchas a thin layer of metal, SiO₂ or SiN. In one embodiment, maskingmaterial is applied to the side walls of relief features by angledevaporative or sputtering deposition techniques combined with samplerotation to ensure deposition all exposed side walls. As shown in FIG.18C, the exposed Si surface is isotropically etched using wet or dryetching methods, thereby releasing the relief features and generatingprintable semiconductor elements, preferably having smooth surfaces.Isotropic etching of the silicon may be achieved using a HNO₃:NH₄F:H₂O64:3:33 solution. Advantages of this fabrication method are therelatively low cost of the silicon substrate starting materials and theability to reuse the mother substrate after planarization (ECMP).

FIG. 18D shows yet another exemplary method of fabricating printablesemiconductor elements from a bulk silicon substrate, preferably asingle crystalline silicon substrate. In this method, the externalsurface of a bulk silicon substrate is selectively patterned withphotoresist. The patterned substrate surface is anisotropically etched,thereby generating relief features. Next the silicon substrate isannealed, for example by annealing in a quartz furnace at a temperatureof about 1100 degrees Celsius and in nitrogen. Next, the surface of theannealed silicon substrate is patterned by masking selected regions withphotoresist. This patterning step defines the shape and some of thephysical dimensions (e.g. length and width for a ribbon) of theprintable semiconductor elements to be fabricated. As shown in FIG. 18D,the patterned surface of the annealed Si substrate is anisotropicallyetched using wet or dry etching methods, thereby generating printablesemiconductor elements, preferably having smooth surfaces. Advantages ofthis fabrication method are the relatively low cost of the siliconsubstrate starting materials, the ability to reuse the mother substrateafter planarization (ECMP) and the ability to integrate sourceelectrode, drain electrode, gate electrode and dielectric devicecomponents after the annealing step. In addition, wet etching may beused in the first etching step with a 110 silicon wafer.

FIG. 18E shows an exemplary method of fabricating printablesemiconductor elements from an ultra thin silicon substrate. In thismethod, the external surface of an ultra thin silicon substrate isselectively patterned with photoresist. This patterning step defines theshape and some of the physical dimensions (e.g. length and width for aribbon) of the printable semiconductor elements to be fabricated. Thepatterned substrate surface is anisotropically etched through thicknessof the ultra thin silicon substrate, thereby printable semiconductorelements. Ultra thin silicon substrates having thicknesses selected fromthe range of about 10 microns to about 500 microns are preferred forsome applications of this fabrication method. An advantage of thisfabrication method is the relatively low cost of the ultra thin siliconsubstrate starting materials.

FIGS. 18F and 18G show exemplary methods for making printablesemiconductor elements from thin films of polycrystalline silicon. Inthis method, a thin layer of polycrystalline silicon is deposited on asupporting substrate, such as glass or silicon substrate, having asacrificial surface layers, such as a coating comprising SiN or SiO₂.The polycrystalline thin film is then annealed, and selective regions ofa the exposed surface are selectively masked by depositing a maskmaterial, such as a thin films comprising a metals, SiO₂ or SiN. Thismasking step defines shape and some of the physical dimensions (e.g.length and width for a ribbon) of the printable semiconductor elementsto be fabricated. The patterned surface is anisotropically etched byeither dry or wet chemical etching methods, generating relief featuresof silicon supported by the sacrificial layer, preferably relieffeatures having smooth side walls. Isotropically etching the sacrificiallayer releases the relief features, thereby generating printablesemiconductor elements. An advantage of this fabrication method is thatthe supporting substrate can be cleaned an reused. Alternatively, a thinlayer of polycrystalline silicon may be deposited directly on a SiO₂substrate. As shown in FIG. 18G, similar annealing, patterning,anisotropic etching and lift off steps may be used to generate printablesemiconductor elements. Optionally, source electrode, gain electrode,gate electrode, dielectric element or any combinations of these may beintegrated into the semiconductor element prior to lift off in either ofthese methods.

FIGS. 18H(1) and 18H(2) illustrate a method for making singlecrystalline semiconductor films using printable semiconductor elementsof the present invention. As shown in FIG. 18H(1), an amorphous orpoly-crystalline semiconductor thin film is prepared on the surface of asubstrate comprising an insulating material, such as SiO₂. The thinamorphous or poly-crystalline semiconductor film may prepared by anymeans known in the art including, but not limited to, depositiontechniques such as vapor deposition or sputtering deposition. Alsoreferring to FIG. 18H(1), a printable semiconductor element comprising asingle crystalline semiconductor structure is transferred onto thesurface of the substrate covered with the thin amorphous orpoly-crystalline semiconductor film. Use of a single crystallinesemiconductor structure having one long lateral dimension is preferredfor some applications of this method. The present invention alsoincludes methods wherein the printable semiconductor element comprisinga single crystalline semiconductor structure is transferred onto thesubstrate surface prior to deposition of the amorphous orpoly-crystalline semiconductor thin film.

As shown in FIG. 18H(2), the thin amorphous or poly-crystallinesemiconductor film is annealed while in contact with the singlecrystalline semiconductor structure, for example by annealing at hightemperatures such as temperatures greater than 1000 degrees Celsius. Inthis embodiment of the present invention, the single crystallinesemiconductor structure acts as a seed promoting a phase transitionthroughout the thin film from an amorphous or poly-crystalline state toa well organized single crystalline state. As shown in FIG. 18H(2), thephase transition follows a front of a high temperature gradient whichmoves across the entire surface of the wafer. Different high temperaturefurnaces or focused optical systems may be used to produce thetemperature gradient necessary to obtain an efficient phase conversionof the semiconductor thin film. An advantage of this process is that itmay significantly reduce the cost of producing single crystallinesemiconductor thin films, such as single crystalline silicon orgermanium films.

FIG. 18I shows an exemplary method of fabricating printablesemiconductor elements comprising micro-wires from GaAs substrate. Asshown in this figure, the exposed surface of the GaAs substrate ispatterned with mask material, such as photoresist. Patterning may beachieved by microcontact or nanocontact printing or via conventionalphotolithography, as shown in FIG. 18E. The patterned surface isanisotropically etched using wet etching methods. In the example methodshown, re-entrant profiles of side walls are obtained using aH₃PO₄—H₂O₂—H₂O solution, and the relief features formed are etchinguntil they are released from the GaAs substrate, thereby generating GaAsmicro-wires. As shown, the photoresist layer may be removed by washingwith acetone and exposure to O₂ reactive ion etching. An advantage ofthis technique is that the GaAs substrate may be reused afterplanarization (ECMP). This technique may also be used to fabricatemicro-wires from an InP substrate.

FIG. 18J shows an alternative method for fabricating printablesemiconductor elements comprising single crystalline silicon ribbons.The starting material in this method is a Si (110) wafer. As shown inFIG. 18J, an exterior surface of the Si (110) wafer is selectedpatterned with thin films of SiO₂ which serves as a mask duringprocessing. This masking step defines shape and some of the physicaldimensions (e.g. length and width for a ribbon) of the printablesemiconductor elements to be fabricated. The exposed (i.e. unmasked)surface of the Si (110) wafer is then isotropically etched by either dryor wet chemical etching methods. This processing step generates relieffeatures of silicon, preferably having smooth side walls separated by aseries of trench have a selected depth. The silicon relief features arethen released from the Si (110) wafer by isotropic etching and liftoffprocessing, thereby generating printable semiconductor elements.Optionally, source electrode, gain electrode, gate electrode, dielectricelement or any combinations of these may be integrated into thesemiconductor element prior to lift off. An advantage of thisfabrication method is that the mother substrate can be cleaned andre-used. FIG. 18J also shows SET micrographs of Si (110) at variouspoints in the processing method.

FIG. 18K shows an alternative method for fabricating printablesemiconductor elements comprising single crystalline silicon ribbons.The starting material in this method is a Si (111) wafer. The Si (111)wafer is selective isotropically etched for example using a combinationof conventional photolithography masking and wet etching methods. Thisprocessing step generates relief features of silicon. As shown in FIG.18K, the side walls, surface or both of the silicon relief features arecoated using a passivation process. Printable single crystalline siliconribbons are released from the Si (111) wafer by isotropic etching andliftoff processing. FIG. 18J also shows SET micrograph of singlecrystalline silicon ribbons generated by this method just prior toliftoff.

Example 4: Methods of Making Semiconductor Nanowires and Microwires

It is a goal of the present invention to provide methods of makingsemiconductor nanowires and microwires having good mechanical andelectrical properties allowing for their use in a wide variety ofdevices, device components and device settings. It is further a goal ofthe present invention to provide methods of assembling nanowires andmicrowires to construct selected single layer structures, multilayerstructures and functional devices comprising these elements. To assessthe utility of the methods of the present invention, nanowires andmicrowires of GaAs and InP were fabricate and evaluated with respect totheir electrical conductivity and mechanical flexibility in a range ofdevice configurations. In addition, the ability of the present methodsto assemble large numbers of nanowires and microwires in well definedpositions and orientations corresponding to large substrate surfaceareas was evaluated by fabricating a number of complex nano/microwireassemblies comprising single layer structures and multilayer structures.The present methods of making and assembling GaAs and InP nanowires andmicrowires were demonstrated to provide excellent control over wirewidth, length and spatial orientation. In addition, the GaAs and InPnanowires and microwires fabricated exhibited good mechanical andelectrical properties when integrated into microelectronic devices.

FIG. 19 provides a schematic diagram illustrating the steps of anexemplary method of generating and transferring nanowire arrays of GaAsto a substrate, such as plastic substrate comprising poly(ethyleneterephthalate) (PET) sheet coated with a thin layer of curedpolyurethane (PU). As shown in FIG. 19, the process begins with a pieceof GaAs wafer with its surface oriented along the (100) direction(American Xtal Technology, Fremont, Calif.). Defining a etch mask ofSiO₂ in the form of lines oriented along the (0 1 1) direction preparesthe structure for anisotropic etching using an aqueous solution of H₃PO₄and H₂O₂ comprising H₃PO₄ (85 wt %):H₂O₂ (30 wt %):H₂O=1:13:12 in volume(step i in FIG. 19). This etching chemistry, when applied in thisfashion, exhibits high anisotropy, thereby generating sharply definedreverse mesa-shaped profiles of GaAs under the SiO₂ mask stripes. Forsufficient etching times, the two side walls of each reverse mesaintersect, resulting in the formation of a wire with triangular crosssection. This triangular cross section is exemplified in the top insetof panel A (left side) of FIG. 19.

In one embodiment, the patterned SiO₂ lines are surrounded by bulk SiO₂film, which results in both ends of each GaAs wire being connected tothe mother wafer. This connection confines the wires and preserves thespatial orientation and layout as defined by the pattern of SiO₂. FIG.20A provides a scanning electron micrograph of free-standing GaAs wiresobtained from GaAs wafer patterned with isolated SiO₂ lines. It isworthy of note that lateral undercutting of GaAs occurs along with thevertical etching, resulting in the ability to decrease the width ofresultant GaAs wires down to nanometer scale even with SiO₂ lines thathave micron widths.

GaAs wire arrays prepared by the present methods may be transfer printedto plastic sheets with retention of the orientation and relativeposition of individual wires in the array. In the embodiment illustratein FIG. 19, a conformable elastomeric transfer element, such as a flatpiece of poly(dimethylsiloxane), or PDMS, Sylgard 184, A/B=1:10, (DowCorning), is placed on the GaAs wafer to pick up the wires (as shown instep ii of FIG. 19). In this embodiment, relatively strong bondingbetween PDMS sheet and the SiO₂ mask layer is required to break thecrystalline connections to the underlying substrate at the ends ofwires.

Cleaning the PDMS stamp and GaAs wafer having the SiO₂ mask with a weakoxygen plasma promotes formation of covalent siloxane (Si—O—Si) bondsbetween PDMS and SiO₂ by a condensation reaction (see the middle insetof FIG. 19). The present invention includes methods, therefore, whereinthe elastomeric transfer element, semiconductor wafer having the SiO₂mask or both are exposed to a weak oxygen plasma to provide effectiveand mechanically strong transfer of the semiconductor wafer having theSiO₂ mask to the elastomeric transfer element. The density of bonds overthe interface is proportional to the number of —O_(n)Si(OH)_(4-n) on thePDMS surface, which is highly dependent on the intensity of oxygenplasma and the treatment time. Treatment with a strong plasma for a longtime can induce bonding that is too strong to release the wires fromPDMS to the desired plastic substrates. Controlled experiments indicatethat the PDMS and SiO₂ coated GaAs wafer treated by the plasma generatedfrom O₂ at pressure of 10 mTorr, flow rate of 10 sccm, and powerintensity of 10 W (Uniaxis 790, Plasma-Therm Reactive Ion EtchingSystem) for 3 and 60 s, respectively, generated the best results. Inthese embodiments, the interaction between the e-beam evaporated SiO₂mask layer and GaAs is strong enough to prevent delamination during thetransfer process. Peeling the PDMS stamp away from the GaAs substrateafter leaving it in contact with the GaAs wafer having the SiO₂ mask for˜2 hrs lifts off all of the wires (as illustrated in step iii of FIG.19).

The present methods make the fabrication and assembly of large numbersof nanowires and/or microwires practically feasible. For example, theGaAs wafer after the transfer step (step iii of FIG. 19) can be polishedto regenerate a flat surface for another run of wire fabrication (stepiv of FIG. 19). The combination of wafer polishing with wire fabricationas described above makes it possible to generate a huge number of GaAswires from a single piece of wafer. For example, one piece of GaAs waferwith diameter of 10 cm and thickness of 450 μm (commercially availablefrom American Xtal Technology) can generate enough wires (˜2.2 billionswires with widths of ˜400 nm and lengths of 100 μm) to densely cover theentire surface of a plastic substrate with an area of 1.76 m² if onecycle of anisotropic etching and polishing consumes 2 μm GaAs inthickness. These conditions are typical of the results described in thepresent example. Accordingly, such repetitive application of wirefabrication followed by wafer polishing steps enables highly costeffective use of the bulk wafers.

As shown in steps v and vi of FIG. 19, the GaAs wires having SiO₂ maskelements can be effectively transferred to a substrate, such as aplastic substrate having an adhesive layer on its outer surface. In oneembodiment, the PDMS stamp with bonded GaAs wires is exposed to ambientenvironment for one day or rinsed it with ethanol to reconstitute thePDMS surface to its native, hydrophobic status. This hydrophobicproperty of the PDMS surface substantially prevents the PDMS fromstrongly interacting with adhesives that are normally hydrophilic. Whenthe recovered PDMS stamp is placed against an adhesive layer, such as aPU layer (obtained from Nolarland products, Cranbury, N.J.) spin-coatedonto a plastic substrate (e.g., PET of ˜175 μm in thickness, Mylar film,Southwall Technologies, Palo Alto, Calif.), only the GaAs wires attachedto SiO₂ mask stripes are wettable to the adhesive. The thickness of PUlayer can be varied from 1 to tens of microns by controlling the spinspeed. Illuminating the sample with an ultraviolet lamp (Model B 100 AP,Black-Ray, Upland, Calif.) for 1 hr cures the PU layer and forms astrong bond between the cured PU and the GaAs wires and SiO₂ maskstripes and between the cured PU and the underlying PET sheet (step v inFIG. 19). Peeling away the PDMS stamp leaves the GaAs wires and SiO₂stripes embedded in the matrix of cured PU with preservation of orderand crystallographic orientation similar to those of wires prior tolift-off (step vi in FIG. 19). The separation of SiO₂ from PDMS stamp isenabled by two effects: i) moderate adhesion strength associated withsparse siloxane bonds at the interface between PDMS and SiO₂ whichfurther weaken during the process of reconstituting PMDS surface; andultra-thin layer of SiO₂ (with thickness of several nanometers) thatremains on the PDMS after cohesive failure of the SiO₂, which can beamorphous, incompact and fragile. Immersing the plastic sheet in asolution of buffered oxide etchant (BOE, NH₄F (40 wt %): HF (49 wt%)=10:1) for 15 min removes the SiO₂ mask stripes, leaving the clean(100) top surfaces of the GaAs nanowires (step vii of FIG. 19) facingout.

This simple ‘top down’ approach to the fabrication and dry printing ofGaAs wire arrays offers many advantages. For example, the geometries(i.e., length, width and shape) of the wires and their spatialorganization can be defined by the initial lithographic step to satisfythe design of the desired electronic or optical end application. Thetransfer printing technique can generate yield as high as 100% withpreservation of the patterns defined by the lithography. The welloriented crystallographic facets of the transferred wires (i.e., the top(100) surface) on plastic substrates provide an extremely flat topsurface (having a flatness similar to that of original wafer), which isvery useful for device fabrication. Furthermore, the SiO₂ mask stripesprevent the top surfaces of GaAs wires from becoming contaminated byorganics, such as PDMS, PU and solvents used in the processing.Embedding the GaAs wires in cured PU immobilizes them, therebypreventing them from moving in lateral or vertical directions,especially when the plastic substrates are bent or twisted. it isimportant to note that PU and PET are only examples of materials usablein the present invention. Accordingly, it will be understood by personsskilled in that art that other adhesives, for example NEA 155 (Norland®)and other types of plastic sheets, for example Kapton® or polyimidefilm, can be used in the methods of the present invention.

Unlike ‘bottom up’ approaches of the prior art, the ‘top down’ processesof the present invention can generate GaAs nanowires with uniformlengths from several microns up to tens of centimeters (i.e., thediameter of original wafers). FIG. 20A shows an SEM image offree-standing GaAs nanowires with widths of ˜400 nm and lengths of 2 cmwhich were randomly assembled on a mother wafer. The long nanowiresformed curved structures during drying process, indicating the highdegree of flexibility that is afforded by their narrow widths. As shownin the lower inset of FIG. 20A, the circled nanowire has a bend radiusas small as ˜20 μm, which suggests that nanowires with width of ˜400 nmcould withstand stain of ˜1.3%. The upper inset in FIG. 20A provides ascanning electron micrograph image of the cross section before ananowire lift off, which clearly shows formation of inverse mesaprofiles of GaAs and undercutting from anisotropic etching.

In one aspect of the present invention, the width of the GaAs wires iscontrolled by selectively adjusting the width of SiO₂ mask lines,selectively adjusting the etching time or both. Widths between hundredsof microns and tens of nanometers are attainable using the presentmethods. Controlling etching time provides an easy way to generatenanowires from SiO₂ patterns having micron widths. FIGS. 20B-E showscanning electron micrograph images of individual wires obtained byetching the GaAs wafer patterned with 2 μm wide SiO₂ lines. The wireswere transferred to a PDMS surface using the procedures described aboveto measure precisely the average width of their top surfaces (referredto as w _(wires)). FIG. 20F provides a plot showing the dependence ofthe average width, w _(wires), of the top surfaces of wires fabricatedby the present methods on etching time. This plot indicates that GaAswires with widths down to 50 nm can be obtained using this embodiment ofthe present methods. The linear relationship between width and etchingtime is consistent with previous studies on etching kinetics of GaAs inH₃PO₄—H₂O₂—H₂O solution, i.e., the etching rate was proportional toetching time when the molar ratio between H₂O₂ and H₃PO₄(n_(H2O2)/n_(H3PO4)) was larger than 2.3 and the molar fraction of H₂O(r_(H2O)) was equal to or less than 0.9 (n_(H2O2)/n_(H3PO4) and r_(H2O)of the etchant used in our experiments were 7.8 and 0.9, respectively).The statistical results show that the distributions of the widths of thewires (as determined by averaging along their lengths) was <9% for wireswith widths of ˜50 nm, which is somewhat narrower than the >14%variation in one type of ‘bottom up’ nanowires that has been reported toprovide average widths of ˜16.8 nm.

The scanning electron micrograph images shown in FIGS. 20B-D also showthat the triangular cross sections of the wires is preserved during thethinning process, indicating the etching is highly anisotropic even forthe free-standing GaAs wires. Close observations of the wires show thatthere is some roughness on their side walls. Much of this roughnesscomes directly from the lithographic procedures used to define the SiO₂mask stripes; some is induced by the misalignment of mask lines andetching itself. This roughness determines the width of the smallestcontinuous wires that we could obtain using this embodiment of thepresent invention. As shown in FIG. 20F, the ratio between the widthvariation along individual wires and average wire width (σ:w) was alsohighly dependent on the etching time. Continuous GaAs nanowires can beprepared when the ratio was less than 100%. The curve provided in FIG.20F indicates that the width of nanowires obtained from application ofthis embodiment of the present invention can be decreased down to ˜40nm. Nanowires with different average widths exhibited essentially thesame width variation along individual wires (i.e., ˜40 nm), which wasclose to the width variation along individual SiO₂ mask lines (i.e., ˜36nm). This comparison confirms that the roughness of wire side walls ismainly initiated by the rough edges of SiO₂ mask stripes regardless ofetching times. Accordingly, use of lithographic procedures that reducethe roughness of the mask stripes reduces the roughness on the edges ofthe wires. It is important to note that the transfer printing processdescribed in this example exposes the pristine, ultraflat unetched topsurface of the wires for electrical connection and device fabrication onthe final substrate (i.e. the PET of FIG. 19).

FIGS. 21A-G shows images of a variety of GaAs wire arrays printed onPDMS and PU/PET substrates. The wires in this case have widths of ˜400nm and lengths of ˜100 μm. The corresponding SiO₂ mask lines had widthsof 2 μm and lengths of 100 μm oriented along the (0 1 1) direction onthe (100) GaAs wafers. FIG. 21A is a scanning electron micrograph imagetaken from an GaAs wire array bonded to a flat PDMS stamp via the SiO₂mask layer, indicating that the order of wires was preserved. The insetof FIG. 21A shows the ends of three wires with relatively highermagnification, clearly revealing the breakage at their ends. As shown inFIG. 21B, peeling the PDMS stamp away from the cured PU left a smoothsurface (as smooth as that of the PDMS) with the SiO₂ mask stripesfacing out. As shown in FIG. 21C, etching away the SiO₂ layers with BOEexposes the pristine top surfaces of GaAs wires. FIG. 21D presents anoptical image collected from a PU/PET substrate with embedded GaAswires, indicating that large-area of wire arrays can be routinelyprinted on the PU/PET substrate using the method illustrate FIG. 19.GaAs wire arrays with other patterns (e.g., patches consisting of wireswith different lengths) can also be transferred to PU/PET substrates.

The transfer process is repeated to print multiple layers of GaAs wirearrays on the same PET substrate by spin-coating a new layer of PU.These methods provide important pathways to generating multilayerstructures comprising nanowires and/or microwires. FIGS. 21E and 21Fgive typical images of multilayer structures having double layers ofGaAs wire arrays. In one embodiment, such multilayer structures areobtained by rotating the second layer with different angles (˜90° and˜45° for E and F, respectively) relative to the first layer. FIG. 21Gprovide images of PU/PET substrates having three layers of GaAs wirearray obtained by repeating the printing process on samples shown inFIGS. 21E and 21F. The thickness of the PU layer, which can becontrolled by tuning the spin speed, controls the spacing between thewire arrays. This type of multilayer capability does not, of course,require any form of epitaxial growth, and the PU insulates the arrays indifferent levels. This fabrication capability is useful for a largenumber of device fabrication applications.

The wire fabrication and printing techniques of the present inventioncan be used to generate wire arrays of other semiconductor materials onplastic substrates by using suitable anisotropic etchants. For example,InP wires with triangular cross sections are fabricated by etching an(100) InP wafer having SiO₂ mask lines along (0 1 1) direction in a 1%(v/v) methanol solution of Br₂. FIGS. 22A-C shows scanning electronmicrograph images of an InP wire array on PMDS and PU/PET substrates.These wires were fabricated from an InP wafer patterned with SiO₂ linesof 50 μm in length and 2 μm in width. The wires shown have lengths andwidths of ˜35 μm and ˜1.7 μm, respectively. The etching behavior of InPin methanol solution of Br₂ is significantly different from that of GaAsin aqueous solution of H₃PO₄—H₂O₂ in terms of profile of wire ends andlateral undercutting. For example, the etching process disconnected allthe ends of InP wires from the mother wafer even with an etch mask thatwas similar to the one used in fabrication of GaAs wires (FIG. 21).Furthermore, the degree of undercutting in InP is less than that inGaAs, indicating that InP wires with small widths (less than 500 nm) canbe more easily prepared by using narrow SiO₂ stripes rather than bycontrolling the etching time.

The mechanical flexibility of a simple two terminal diode device madewith GaAs wire arrays (same as those shown in FIG. 21 which werefabricated from Si-doped n-GaAs wafer with carrier density of1.1-5.6×10¹⁷ cm⁻³) on a PU/PET substrate was evaluated by measuring theelectrical properties as a function of bend radius. The structures werefabricated with GaAs wire arrays defined according to the methods ofFIG. 19. Photolithography and metal deposition defined on these wirestwo Schottky contacts made of Ti/Au (5 nm/150 nm) and separated by 10μm. FIG. 23A provides a schematic diagram and image of an exemplary twoterminal diode device comprising GaAs wire arrays. Dipping the substrateinto a concentrated HCl solution for 10 min removed the native oxidelayers on the surfaces of GaAs wires just before deposition ofelectrodes.

FIG. 23B shows the current-voltage (I-V) curves recorded at differentbend radii. These curves all exhibit expected diode characteristics. Thesmall differences among these curves suggest that almost no GaAsnanowires were broken even when the bend radius (R) of substrate was0.95 cm. The strain on the PET surface in this case was ˜0.92%, which isless than that estimated to exist in the free-standing GaAs nanowireshown in the inset of FIG. 20A. These results further confirm that GaAsnanowires generated by the present ‘top down’ fabrication methods areflexible and are able to be integrated with bendable plastic sheets. Wenote that the data show that when the substrate was relaxed after firstbending it, the current was ˜40% smaller than that recorded fromoriginal device before bending. FIG. 23C shows the current-voltage (I-V)curves measured for the two terminal diode device after relaxation afterbending at different bend radii. For the sake of comparison, the blackcurve in FIG. 23C shows the current-voltage curve corresponding to thedevice configuration prior to bending. The lack of variation in the I-Vcharacteristics with bend radius and with multiple bending/unbendingcycles after the first one suggest, however, that the one-time decreasein current might be caused by an initial variation in the properties atthe interface between the electrodes and the wires.

The combined use of traditional photolithography and anisotropicchemical etching with bulk high quality single crystal wafers of thesematerials forms an attractive ‘top down’ route to micro and nanowires ofGaAs and InP with triangular cross sections. The dimensions of the wiresand their organization are selectively adjustable by appropriateselection of the lithography and etching conditions, for example etchingtime. The as-obtained wire arrays on the mother substrates are able tobe effectively transfer printed with high fidelity to plastic substratescoated with a thin layer of adhesive in which the wires are embedded.The mother wafer is able to be re-used after polishing, which enableslarge numbers of wires to be generated from a single wafer. This “dry”transfer printing of ‘top down’ nanowires/micron wires represents a newclass of transfer processes that offer many advantages over “wet”assembly of ‘bottom up’ nanowires in terms of preservation of order andcrystallographic orientation of the wires as well the purity of theiractive surfaces. For macroelectronics applications in particular, wherewires wider than 100-200 nm are useful, the “top down” fabricationmethods of the present invention has many attractive features. Thesystems of wires on plastic substrates demonstrated here illustrateexcellent bendability and significant potential for use in this class ofapplication.

Example 5: Solution Printing Methods for Printable SemiconductorElements

The present invention provides solution printing methods capable oftransferring and assembling printable semiconductor elements over largeareas of many substrates. This aspect of the present invention providescontinuous, high speed fabrication methods applicable to a wide range ofsemiconductor devices and device components.

In one method of this aspect of the present invention, a printablesemiconductor element is provided having a handle element. In thecontext of this description, the term “handle element” refers to acomponent that allows for controlled manipulation of the position and/ororientation of a printable semiconductor element after solution phasedelivery to a substrate surface. In one embodiment, a semiconductorelement is provided having one or more handle elements each comprising alayer of material that is responsive to a magnetic field, electricfield, or both. This aspect of the present invention is useful forproviding methods of aligning, positioning and/or orienting printablesemiconductor elements on substrate surfaces using electrostatic forces,and/or magnetostatic forces. Alternatively, the present inventionprovides methods wherein a semiconductor element is provided having oneor more handle elements each comprising a layer of material that isresponsive to laser induced momentum transfer processes. This aspect ofthe present invention is useful for providing methods of aligning,positioning and/or orienting printable semiconductor elements onsubstrate surfaces by exposing the printable semiconductor elementhaving one or more handle elements to a series of laser pulses (e.g.laser tweezers methods). Alternatively, the present invention providesmethods wherein a semiconductor element is provided having one or morehandle elements each comprising a droplet that is response to forcesgenerated by capillary action. The present invention includes methodsand devices using printable semiconductor elements having one or morehandle elements or one or more different types of handle element, suchas handle elements responsive to different types of fields. Handleelements may be provide in many types of printable semiconductorelements of the present invention including, but not limited to,microstructures, nanostructures, microwires, nanowires, microribbons andnanoribbons.

In this aspect of the present invention, one or more printablesemiconductor elements each having one or more handle elements aredispersed into a solution or carrier fluid and delivered to a substratesurface. Delivery of the mixture of printable semiconductor elements andsolution/carrier fluid distributes the printable semiconductor elementsrandomly about the substrate surface. Next, the semiconductor elementsrandomly distributed on the substrate surface are concertedly moved intoselected positions and orientations on the substrate surface byapplication of forces arising from the presence of handle elements ofthe printable semiconductor elements. This aspect of the presentinvention is useful for aligning printable semiconductor elements havinghandle elements into well order arrays or into positions andorientations corresponding to a selected device or device componentconfiguration. For example, printable semiconductor elements having oneor more handle elements comprising a layer of magnetic material may bemoved into selected positions and orientations on a substrate surface byapplication of a magnetic field having appropriate distributions ofintensities and directions. In this embodiment, a magnetic field havingselected distributions of intensities and directions may be applied bypositioning one or more ferromagnetic elements or electromagneticelements proximate to the substrate (such as positioned behind thesubstrate surface, above the substrate surface and/or along side thesubstrate), thereby generating selected distributions of intensities anddirections which correspond to a desired assembly, pattern or structureof printable semiconductor elements or a selected device or devicecomponent configuration. In this aspect of the present invention,solvent, carrier fluid or both may be removed before, during or afterselective positioning and orientation of printable semiconductorelements via manipulation of handle elements by any means known in theart, including by evaporation or by desorption methods.

FIG. 24 provides a schematic diagram illustrating an exemplary method ofthe present invention for solution printing printable semiconductorelements having handle elements comprising magnetic tags. As shown inFIG. 24, printable semiconductor elements are provided, each of whichhave a plurality of magnetic tags comprising thin nickel layers. In oneembodiment, thin nickel layers are provided on the surface of amicrosized or nanosized semiconductor structure. The use of nickel forhandle elements in this example is only by way of example and anycrystalline or amorphous ferromagnetic material may be used in thesemethods, including but not limited to, Co, Fe, Gd, Dy, MnAs, MnBi, MnSb,CrO₂, MnOFe₂O₃, NiOFe₂O₃, CuOFe₂O₃, MgOFe₂O₃ and amorphous ferromagenticalloys such as transition metal-metalloid alloys.

As shown in step I of the schematic diagram in FIG. 24, a plurality ofprintable semiconductor elements each having handle elements aredispersed into solution and cast onto the surface of a substrate. Thisstep provides the printable semiconductor elements to the substratesurface in a random distribution of positions and orientations. As shownin step II of the schematic diagram in FIG. 24, a magnetic field is thenapplied to the printable semiconductor elements, preferably a magneticfield having selected distributions of intensities and directions. Inthe schematic shown in FIG. 24, a magnetic field having selecteddistributions of intensities and directions is applied by positioningthe magnetic poles of one or more magnets on opposite sides of thesubstrate surface having printable semiconductors thereon. As aferromagnetic material, interaction of the magnetic field and the nickellayers comprising the handle elements generates forces which move theprintable semiconductors into desired positions and orientations on thesubstrate surface. In the embodiment shown in FIG. 24, a magnetic fieldis applied that orients the printable semiconductor elements into a wellordered array characterized by a substantially parallel alignment of thelong sides of the printable semiconductor elements. As shown in step IIIof the schematic diagram in FIG. 24, electrical connections can bedeposited on the ends of printable semiconductor elements comprising theordered array in a manner establishing an electric connection and in amanner preserving the orientations prepared by application of themagnetic field.

FIG. 25 provides several optical images demonstrating the use ofsolution printing methods of the present invention to generate wellorder arrays of microstructures comprising printable semiconductorelements having handle elements comprising thin nickel layers. Theoptical images presented in the panels on the left of FIG. 25 correspondto a substrate surface having printable semiconductors dispersed on asubstrate surface without the application of a magnetic field. As shownin these images, the printable semiconductor elements are randomlydistributed on the substrate surface. The optical images presented inthe panels on the right of FIG. 25 correspond to a substrate surfacehaving printable semiconductors dispersed on a substrate surface uponthe application of a magnetic field. In contrast to the images presentedin the left panels, the optical images corresponding to conditionswherein a magnetic field is applied show that the printablesemiconductor elements are provided in selected orientations andpositions corresponding to a well ordered array. Comparison of theimages presented in the left panels and right panels of FIG. 25 indicatethat application of a magnetic field having selected distributions ofintensities and directions is capable of generating forces that moveindividual printable semiconductor elements into selected positions andorientations.

As will be understood by persons of skill in the art of devicefabrication, the positions and orientations of printable semiconductorelements in the right panels of FIG. 25 are but one example oforientations and positions achievable using the solution printingmethods of the present invention. Selection of appropriate positions ofhandle elements on printable semiconductor elements and selection ofappropriate magnetic fields having selected distributions of intensitiesand directions may be used to generate virtually any distribution ofsemiconductor element positions and orientations.

Example 6: Fabrication of High Performance Single Crystal SiliconTransistors on Flexible Plastic Substrates

It is a goal of the present invention to provide bendablemacroelectronic, microelectronic and/or nanoelectronic devices anddevice components comprising printable, high quality semiconductorelements assembled on flexible substrates. In addition, it is a goal ofthe present invention to provide bendable electronic devices, such asbendable thin film transistors that exhibit field effect mobilities,on-off ratios and threshold voltages similar to or exceeding that ofthin film transistors fabricated by convention high temperatureprocessing methods. Finally, it is a goal of the present invention toprovide bendable electronic devices compatible with efficient highthroughput processing on large areas of flexible substrates at lowertemperatures, such as room temperature processing on plastic substrates.

The ability of the present methods, devices and compositions to provideuseful macroelectronic and/or microelectronic devices and devicecomponents exhibiting high device performance characterizing in bent andplanar configurations was verified by experimental studies. The resultsof these measurements demonstrate that the present invention providesdry transfer contact printing techniques, exhibiting excellentregistration capability, that are capable of assembling bendable thinfilm transistors by depositing a range of high quality semiconductors,including single crystal Si ribbons, Ga—As and InP wires andsingle-walled carbon nanotubes onto plastic substrates. For example, theresults of these experimental studies indicate that bendable thin filmtype transistors comprising spatially well defined arrays of drytransferred printable single crystal silicon elements exhibit highdevice performance characteristics, such as average device effectivemobilities, evaluated in the linear regime, of ˜240 cm²/Vs, andthreshold voltages near 0 V. Further, these studies show that the thinfilm transistors of the present invention exhibit bendability (i.e.strain at which failure occurs) comparable to devices made with organicsemiconductors, and mechanical robustness and flexibility when subjectedto frontward and backward bending.

High performance printed circuitry on large area flexible substratesrepresents a new form of electronics that has wide ranging applicationsin sensors, displays, medical devices and other areas. Fabricating therequired transistors on plastic substrates represents a challenge toachieving these macroelectronic systems. Some approaches that have beenexplored over the last several years are based on modified, lowtemperature versions of the types of process steps used to fabricateconventional silicon based thin film transistors (TFTs) on glass/quartzsubstrates. The high temperatures associated with the directionalsolidification processes developed for producing single-crystal siliconfilms (i.e., zone-melting recrystallization of Si films on SiO₂ using acw laser, a focused lamp, an electron beam, or a graphite-strip heater)make this approach unsuitable for use with plastic substrates. Laserbased approaches have achieved some limited degree of success, althoughuniformity, throughput and use with low cost plastics poses significantcontinuing experimental challenges. Direct full wafer transfer ofpreformed circuits onto plastic substrates yield some useful devices,but this approach is difficult to scale to large areas and it does notretain printing type fabrication sequences that might be important forlow cost, large area macroelectronics. Organic semiconductor materialsprovide an alternative pathway to flexible electronics; wherein theorganic based electronic materials can be naturally integrated, via roomtemperature deposition, with a range of plastic substrates. The organicsemiconductors materials currently known, however, enable only modestdevice mobilities. For example, even high quality single crystals ofthese materials have mobilities in the range of 1-2 cm²/Vs and ˜10-20cm²/Vs for n and p-type devices, respectively.

Other fabrication techniques such as fluidic self assembly separate thehigh temperature steps for producing high mobility materials from thelow temperature processing that is required for building devices onplastic substrates. These methods do not, however, allow efficientcontrol of the organization or location of the deposited objects.

FIG. 26A illustrates the steps used to fabricate exemplary bendable thinfilm transistors devices of the present invention. First,photolithography defined a pattern of photoresist on the surface of asilicon-on-insulator wafer (Soitec unibond SOI with a 100 nm top Silayer and 145 nm buried oxide). This resist served as a mask for dryetching the top silicon layer of the SOI wafer with a SF₆ plasma(Plasmatherm RIE system, 40 sccm SF₆ flow with a chamber base pressureof 50 mTorr, 100 W RF power for 25 s). A concentrated HF solution etchedthe buried oxide and freed (but did not completely float off) theprintable single crystal silicon semiconductor elements from theirsubstrate. A flat piece of poly(dimethylsiloxane) (PDMS) was broughtinto conformal contact with the top surface of the wafer and thencarefully peeled back to pickup the interconnected array of ribbons. Theinteraction between the photoresist and the PDMS was sufficient to bondthe two together for removal, with good efficiency.

An Indium-Tin-Oxide (ITO; thickness ˜100 nm) coatedpoly(ethyleneteraphtalate) (PET; thickness ˜180 μm) plastic sheet servedas the device substrate. Washing it with acetone & isopropopanol,rinsing it with deionized water and then drying it with a stream ofnitrogen cleaned its surface. Treating the ITO with a short oxygenplasma (Plasmatherm RIE system, 20 sccm O₂ flow with a chamber basepressure of 100 mTorr, 50 W RF power for 10 s) promotes adhesion betweenit and a spin cast dielectric layer of epoxy (3000 RPM for 30 s ofMicrochem SU8-5 diluted with 66% of SU8-2000 thinner). This photosensitive epoxy was pre-cured at 50° C. on a hot plate during ˜1 min.Bringing the PDMS with a printable single crystal silicon semiconductorelement on its surface into contact with the warm epoxy layer and thenpeeling back the PDMS led to the transfer of the printable singlecrystal silicon semiconductor element to the epoxy. This result suggeststhat the bonding forces between the silicon and the soft epoxy layer(some of which are mechanical, due to the flow of epoxy around the edgesof the printable single crystal silicon semiconductor elements) arestronger than those between the photoresist and the PDMS stamp. Theepoxy layer was fully cured at 100° C. for 5 min, exposed to UV lightfrom the back side of the transparent substrate for 10 s and then postbaked at 115° C. for 5 min to crosslink the polymer. The photoresistmask (which, conveniently, prevents contamination of the top surface ofthe printable single crystal silicon semiconductor elements during thetransfer steps) was dissolved with acetone and the sample was thenabundantly rinsed with deionized water.

Source and drain electrodes were formed with Ti (˜70 nm; Temescal e-beamevaporator) deposited on the top surface of the printable single crystalsilicon semiconductor elements. Etching (1:1:10 HF:H₂O₂:DI for ˜2 s)through a photoresist mask (Shipley S1818) patterned on the Ti definedthe geometry of these electrodes. The last step of the fabricationinvolved dry etching (SF₆ using the RIE parameters given above) througha photoresist mask to define islands of silicon at the locations of thedevices. FIG. 26B presents a schematic illustration of the bottom gatedevice configuration of the thin film transistor together with high andlow magnification optical images of part of the device array.

FIG. 27A presents current voltage characteristics of a bendable thinfilm transistor of the present invention that shows an effective devicemobility of 140 cm²/Vs in the saturation regime and 260 cm²/Vs in thelinear regime, as evaluated by application of standard field effecttransistor models that ignore the effects of contacts. The highresistance (˜90 S/cm) of the Schottky contacts in these devices,however, has a significant effect on the device response. FIG. 27Bpresents transfer characteristics of several devices, plotted on linear(left axis) and logarithmic (right axis) scales. The plot in the insetshows that the threshold voltages have a narrow distribution near 0 V.Small (<4% in current for a ±10 V cycle) hysteresis in the transfercharacteristics indicates a low density of trapped charge at theinterface between the silicon (with native oxide) and the epoxydielectric. The small values (≤13 V·nF/dec·cm²) of the normalizedsubthreshold slopes confirm the good quality of this interface, whichmay be governed primarily by the interface between the silicon and itsnative oxide. FIG. 27C shows the distribution of the linear effectivemobilities of several bendable thin film transistors fabricate by thepresent methods. A Gaussian fit indicates a center value of 240 cm²/Vswith a standard deviation of 30 cm²/Vs. Some of the low values areassociated with visible defects in the electrodes or other components ofthe devices. The uniformity of the epoxy dielectric was investigated bybuilding, using the same substrate and methods used to prepare thetransistor gate dielectric, an array of 256 (200×200 μm) squarecapacitors. The inset in FIG. 27C shows the measured capacitance values.A Gaussian fit indicates a standard deviation lower than 2% confirmingthe excellent electrical and physical properties uniformity of the epoxylayer. Capacitance measurements carried out at various frequencies(between 1 kHz and 1 MHz) indicted a small (<3%) frequency-dependence ofthe dielectric constant.

The mechanical flexibility and robustness of the bendable thin filmtransistors of the present invention were investigated by performingfrontward and backward bending tests. FIG. 28A presents ahigh-resolution scanning electron micrograph of solution cast ribbons(left inset) illustrating the remarkable flexibility of the printablesingle crystal silicon semiconductor elements. The right inset in FIG.28A shows a picture of the experimental setup used to bend the bendablethin film transistors evaluated in this study. To maximize the straininduced in the thin film transistors when the plastic sheet is bent, arelatively thick (˜180 μm) plastic substrate was used in these studies.FIG. 28B shows the small (˜<1%) linear variation of the epoxy dielectriccapacitance when subject to tensile and compressive strains (see topinset). The bending radius and strain values where computed using afinite element model of the buckling sheet. Comparisons of the bendingprofiles of the buckling sheet (for several bending radius) to theprofiles obtained with the finite element method confirmed the accuracyof the simulations. The lower inset in FIG. 28B presents the variationof the saturation current of a device measured for a gate and drain biasvoltages of both 4 V. The maximum value of the tensile strain at whichthe bendable thin film transistor can be operated seems to be limited bythe failure of the ITO gate electrode (which fails a tensile strainvalue of ˜−0.9%). The bendable thin film transistor operate well even atcompressive strains as high as 1.4%. This level of bendability iscomparable to that recently reported for organic transistors based onpentacene. Failure of the present bendable thin film transistors islikely to occur only at very high strains, as Takahiro et al. recentlydemonstrated that micron-sized single crystal silicon objects etchedfrom the top layer of a SOI wafer can withstand remarkably high tensilestress (>6%) [T. Namazu, Y. Isono, and T. Tanaka J. MEMS 9, 450 (2000)]

The cause of the modest variation in output current with strain in ourdevices is not entirely known, as the known variation in mobility withstrain contributes to, but does not fully account for, these changes.Devices of the type that we describe in this example enable newopportunities to investigate the charge transport in mechanicallystrained silicon at strain values not easily reached when bulk Si wafersare bent.

In summary, this Example demonstrates the high device performance andbeneficial mechanical properties of bendable single crystal silicontransistors formed on plastic substrates by the simple and efficientparallel printing process for silicon provided by the present invention.To the best of our knowledge, the performance of these devices exceedsthat of the best devices (silicon based or otherwise) exhibiting asimilar degree of mechanical bendability. The top down control over theshapes, physical dimensions and composition (e.g. doped or not doped) ofthe printable silicon semiconductor elements, and the printing techniqueprovide significant advantages compared to other approaches. Inaddition, the mechanical flexibility of the resulting devices isexcellent. Further, these same general approaches are applicable toother inorganic semiconductors (e.g. GaAs, GaN, etc.) and can be used tofabricate a wide range of flexible microelectronic and macroelectronicdevices and device components, such as solar cells, diodes, lightemitting diodes, complementary logic circuits, information storagedevices, bipolar junction transistors, and FET transistors. Therefore,the methods and device of the present invention are useful for anenormous range of fabrication applications for making flexibleelectronic products.

Example 7: Printable Heterogeneous Semiconductor Elements and DevicesComprising Printable Heterogeneous Semiconductor Elements

The present invention provides heterogeneous printable semiconductorelements comprising multimaterial elements, and related devices anddevice components. Printable heterogeneous semiconductor elements ofthis example comprise a semiconductor layer having a dopant with aselected spatial distribution, and provide enhanced functionality in arange of macroelectronic, microelectronic and/or nanoelectronic devices.

The ability of the present methods to fabricate heterogeneous printablesemiconductor elements exhibiting useful electronic properties wasverified by experimental studies. In addition, the applicability of thepresent methods to assembling printable elements into functional deviceswas demonstrated by fabrication of flexible thin film transistorscomprising printable single crystal silicon semiconductor elementshaving integrated doped regions for contacts.

Large area, mechanically flexible electronic systems, known asmacroelectronics, are attractive for a range of applications in consumerelectronics, sensors, medical devices and other areas. A variety oforganic, inorganic and organic/inorganic hybrid materials have beenexplored as semiconductors for these systems. Use of the present“top-down” technology approach for generating single crystal siliconmicro/nanoelements (wires, ribbons, platelets, etc., which are refer tocollectively as printable silicon semiconductor elements) is analternative approach that has been demonstrated as useful forfabricating high performance thin film transistors on flexiblesubstrates. This fabrication approach has also been demonstrated asapplicable to other important semiconductor materials, such as GaAs,InP, GaN, and carbon nanotubes.

An important characteristic of the present approach is its use of highquality, wafer-based sources of semiconductor materials, which are grownand processed separately from subsequent device assembly steps. Separatesemiconductor processing and assembly steps allow for device assembly atrelatively low temperatures (e.g. room temperature ±30 degrees Celsius)compatible with most flexible device substrates, such as plasticsubstrates. The present invention includes methods wherein high qualitysemiconductors are not only grown, but are also processes in other waysin fabrication steps independent from subsequent fabrication stepsinvolving assembly of printable semiconductor elements on flexiblesubstrates. In one embodiment, the present invention includes methodswherein dopants are introduced into the semiconductor during hightemperature processing, and the resulting doped semiconductor materialis subsequently used to generate printable heterogeneous semiconductorelements which can be assembled into a variety of useful electronicdevices. Processing steps useful in for doping semiconductors includehigh temperature processing and processing wherein dopants areintroduced in a manner providing control over their spatial distributionin one two or three dimensions (i.e. control over the area ofimplantation and depth of implantation). In one method, thesemiconductor is selectively contact doped by spin on doping proceduresperformed at the wafer fabrication level in steps that are performedindependently from the low temperature substrates. Contact dopingprovides precise control over the spatial distribution of dopant in thesemiconductor material and, therefore, subsequent patterning and etchingsteps allow fabrication of high quality printable heterogeneoussemiconductor elements having integrated doped regions. The solutionprinting and dry transfer contact printing methods are ideally suitedfor assembling these printable heterogeneous elements into devices, suchas thin film transistors, exhibiting the excellent device performanceand excellent bendability that can be achieved.

FIG. 29A presents a schematic representation of a fabrication processfor generating transistors comprising printable heterogeneoussemiconductor elements on a PET substrate. In this embodiment, theprintable heterogeneous semiconductor element comprises crystal siliconwith doped source (S) and drain (D) contacts. The approach illustratedin FIG. 29A uses a solution processable spin-on dopant (SOD) to dopeselected regions of the top silicon layer of a silicon-on-insulatorwafer (SOI; Soitec unibond with a 100 nm top Si layer and 200 nm buriedoxide). Therefore, a spin-on dopant (SOD) provides the phosphorus dopantand a spin-on glass (SOG) serves as a mask to control where dopantdiffuses into the silicon. This doped SOI provides the source ofprintable heterogeneous semiconductor elements.

To produce the printable heterogeneous semiconductor elements we firstspin cast a spin-on glass (SOG) solution (Filmtronic) onto a SOI waferand exposed it to rapid thermal annealing (RTA) at 700° C. for 4 minutesto form a uniform film (300 nm thickness). Etching (6:1 buffered oxideenchant (BOE) for 50 seconds) through a lithographically patterned layerof photoresist (Shipley 1805) opened source and drain windows in theSOG. After stripping the resist, we uniformly deposited a phosphorouscontaining SOD (Filmtronic) by spin casting. RTA at 950° C. for 5seconds caused the phosphorous from the SOD to diffuse through thelithographically defined openings in the SOG and into the underlyingsilicon. The SOG blocked diffusion in the other areas. The wafer wasrapidly cooled to room temperature, immersed in BOE for 90 seconds toremove both the SOG and SOD and then thoroughly washed with DI water tocomplete the doping process.

The printable heterogeneous semiconductor elements were assembled ontoplastic substrates of PET coated with indium tin oxide (ITO; 100 nm,gate electrode) and epoxy (SU8; 600 nm, gate dielectric) using thepresent methods. The epoxy not only provides the dielectric, but alsofacilitates the transfer printing of the printable heterogeneoussemiconductor elements. Source and drain electrodes of Ti (100 nm) wereformed on the doped contact regions by an aligned photolithography stepfollowed by etch back. FIG. 29B shows optical images of several deviceshaving heterogeneous printable semiconductor elements fabricated usingthe present techniques.

We estimated the doping levels and the contact resistances using astandard transfer line model (TLM). In particular, we measuredresistances between Ti contact pads with separations (L) between 5 and100 microns and widths (W) of 200 microns on printable heterogeneoussemiconductor elements comprising uniformly doped crystalline siliconthat were printed onto a plastic substrate were measured. The inset inFIG. 30A shows images of the arrangement of printable heterogeneoussemiconductor elements and contact pads used to characterize the contactresistances. The linear current (I) vs. voltage curves (V) (not shown)indicate that the contacts are ohmic and that the doping level is high.The dependence of the resistance on L can be described byR_(total)=2R_(c)+(R_(s)/W)L, where R_(total) (=V/I) is the resistancebetween two contact pads, R_(c) is the contact resistance, and R_(s) isthe sheet resistance. FIG. 30A shows a plot of the normalizedresistance, R_(total)W, _(as) as a function of L. Linear fitting ofR_(total)W gives R_(s)=228±5 Ω/sq, and R_(c)W˜1.7±0.05 Ω·cm. The valueof normalized contact resistance R_(c)W is more than an order ofmagnitude lower than what we observed for undoped printable singlecrystal silicon semiconductor elements processed in a similar manner.The resistivity is about 2.3×10⁻³ Ω·cm, which corresponds to a dopinglevel of 10¹⁹/cm³ if we assume, for simplicity, that the doping isuniform through the 100 nm doped printable silicon semiconductorelement. FIG. 30B shows Time-of-Flight Secondary Ion Mass Spectroscopy(TOF-SIMS) measurements that show the use of patterned SOG as adiffusion barrier (See, Schematic in FIG. 29A) localizes the dopants todesired regions in the silicon. In the image shown in FIG. 30B, thebright red color indicates high phosphorous concentration.

FIGS. 31A-D shows measurements corresponding to transistors comprisingprintable contact doped silicon semiconductor elements on anepoxy/ITO/PET substrate. FIG. 31A plots the current-voltagecharacteristics of a device (L=7 μm, W=200 μm) of the present invention.The effective device mobility (μ_(eff)) is ˜240 cm²/Vs in the linearregime and ˜230 cm²/Vs in the saturation regime, as determined byapplication of standard field-effect transistor models. FIG. 31B showstransfer characteristics of devices of the present invention withchannel lengths between 2 μm and 97 μm and channel widths of 200 μm. TheON to OFF current ratios in all cases are ˜10⁴. The threshold voltagesvary from ˜2 V to ˜0 V, monotonically from L=97 μm to 2 μm. FIG. 31Cpresents the resistance of the devices measured in ON state (R_(on)) atsmall drain voltages, and multiplied by W, as a function of L atdifferent gate voltages. Linear fits of R_(on)W vs. L at each gatevoltage provide information about both intrinsic device mobility andcontact resistance. In this simple model, R_(on) consists of the seriesaddition of the channel resistance (which is proportional to L) and thecombined contact resistance R_(c) associated with the source and drainelectrodes. FIG. 31C shows that R_(c), as determined from the interceptsof linear fitting, is negligible compared to channel resistance for allchannel lengths evaluated. The inset in FIG. 31C in shows the variationof sheet conductance, as determined from the reciprocal of the slope oflinear fitting in FIG. 31C, with gate voltage. The linear fit to thesedata, as shown in the inset of FIG. 31C, gives an intrinsic devicemobility of ˜270 cm²/Vs, and an intrinsic threshold voltage of ˜2 V.

FIG. 31D compares the effective mobilities, μ_(eff), of transistorshaving undoped and contact-doped printable single crystal siliconsemiconductor elements evaluated directly from transfer characteristicsmeasured in the linear regime (i.e. contact effects are not subtracted).For the undoped devices, μ_(eff) decreases rapidly from 200 cm²/Vs to 50cm²/Vs with decreasing the channel length L from 100 microns to 5microns. The contacts begin to dominate device behavior at channellengths below 50 microns. In the contact doped case, the mobilities arearound 270 cm²/Vs, with <20% variation with channel length over thisrange, which is in consistent with the intrinsic device mobilitydetermined from inset of FIG. 31C. These data provide additionalevidence that these devices show negligible effects of contactresistance. We note that, in addition to the different mobilities, wenotice that the devices with doped contacts are more stable, moreuniform in their properties and less sensitive to processing conditionsthan those with undoped contacts.

Mechanical flexibility is an important characteristic of devices of thistype. We performed systematic bending tests on the contact-doped μs-Sitransistors, with bending directions that place the devices incompression and in tension. We also carried out some fatigue tests. Thedetails of the experimental set-up is provided in Example 6. FIG. 32Ashows the change of the effective device mobility, normalized by thevalue in the unbent state, μ_(0eff), as a function of strain (or bendingradius). Negative and positive strains correspond to tension andcompression, respectively. For this range of strains (corresponding tobend radii down to ˜1 cm for the 200 micron thick substrate), weobserved only small (<20% in most cases) changes in μ_(eff)/μ_(0eff),the threshold voltage and the on/off ratio. This level of mechanicalflexibility is comparable to that reported for organic and a-Sitransistors on plastic substrates. FIG. 32B presents normalizedeffective mobilities μ_(eff)/μ_(0eff) after several hundred bendingcycles (to a radius of 9.2 mm) that cause compressive strain at thedevice to vary between 0 and 0.98%. Little change in the properties ofthe devices was observed; after 350 cycles, the μ_(eff)/μ_(0eff), thethreshold voltage and the on/off ratio change by less than 20%. Theseresults indicate good fatigue stability of the present transistorscomprising printable heterogeneous semiconductor elements.

This example demonstrates the usefulness of spin-on dopant processes forcontact-doped printable single crystal silicon semiconductor elements intransistors on plastic substrates. Scaling analysis indicates that thepresent process yields devices that are not contact limited, whichdemonstrates the applicability of the present methods for fabricatinghigh frequency silicon devices on plastic substrates. This feature,combined with the remarkably good mechanical flexibility and fatiguestability of the devices, make this contact doped printableheterogeneous semiconductor approach a valuable route to a wide varietyof flexible macroelectronic, microelectronic and/or nanoelectronicsystems.

The present invention also provides heterogeneous integration methodsfor integrating printable semiconductor elements into a range of devicesand device configurations. This aspect of the present invention providesfabrication pathways for making a wide range of devices whereindisparate classes of materials are assembled and interconnected on thesame platform. Heterogeneous integration methods of the presentinvention utilize solution printing and/or dry transfer contact printingto combining two or more different materials in a manner establishingtheir electrical, optical and/or mechanical interconnectivity. Printablesemiconductor elements of the present invention may be integrated withdifferent semiconductor materials or other classes of materials,including dielectrics, conductors, ceramics, glasses and polymericmaterials.

In one embodiment of this concept, heterogeneous integration involvestransferring and interconnecting a printable semiconductor element to asemiconductor chip having a different composition, for example toassemble a system on a chip type device. In another embodiment, aplurality free standing devices and/or device components are fabricatedon different kinds of semiconductor wafers (e.g. silicon wafers and GaNwafers) and subsequently integrated together on the same receivingsubstrate, such as a receiving wafer. In yet another embodiment,heterogeneous integration involves introducing one or more printablesemiconductor elements into preformed complex integrated circuits byassembling the printable elements in specific orientations andeffectively interconnecting the printable elements with other componentscomprising the integrated circuit. Heterogeneous integration methods ofthe present invention may employ a large number of other techniques forassembling and interconnecting microscale and/or nanoscale printablesemiconductor elements known in the art including, but not limited to,wafer bonding methods, use of adhesives and intermediate bonding layers,annealing steps (high and low temperature anneal), treatment to stripoxide outer coating, semiconductor doping techniques, photolithographyand additive multilayer processing via successive thin film layertransfer.

FIG. 33 provides a SEM image a composite semiconductor structurecomprising gallium nitride microstructures direct-bonded onto a siliconwafer (1 0 0) fabricated using a heterogeneous integration method of thepresent invention. To fabricate the composite semiconductor structureshown in FIG. 33, printable semiconductor elements comprising GaN weremicromachined from a GaN on silicon (111) wafer using inductivelycoupled plasma etching and released from the silicon using ananisotropic wet etch in hot aqueous KOH (100 degrees Celsius). Theprintable GaN elements were removed from the mother chip and printedonto a receiving silicon chip by dry transfer contact printing using aPDMS stamp. Bonding between the printable GaN elements and the siliconchip is provided by attractive intermolecular forces does not requireuse of an adhesive layer. The SEM image provided in FIG. 33 shows thatprintable semiconductor elements and transfer printing assembly methodsof the present invention are capable of heterogeneous integration ofdifferent semiconductor materials.

Example 8: Fabrication of High Performance Solar Cells Having PrintableSemiconductor Elements

It is a goal of the present invention to provide methods of making solarcells, solar cell arrays and integrated electronic devices having solarcells on large areas of substrates having a range of compositions,including flexible plastic substrates. In addition, it is a goal of thepresent invention to provide heterogeneous printable semiconductorelements capable of providing P-N junctions in solar cells exhibitingphotodiode responses comparable to solar cells fabricated by conventionhigh temperature processing methods.

The ability of printable semiconductor elements of the present inventionto provide heterogeneous printable semiconductor elements comprising P-Njunctions with high quality P-N layer interfaces in solar cells wasverified by experimental studies. Solar cells were fabricated using twodifferent fabrication pathways for making P-N junctions, and thephotodiode responses of devices made by these pathways were evaluated.The experimental results provided in this example demonstrate thatprintable heterogeneous semiconductor elements and related assemblymethods of the present invention are useful for providing high qualityP-N junctions in solar cells.

FIG. 34A provides a process flow diagram schematically illustratingprocessing steps in a fabrication pathway for making a solar cellcomprising a printable P-N junction. As shown in FIG. 34A, a highquality semiconductor material, such as a single crystalline siliconwafer, is provided and processed in a manner generating an N dopedsemiconductor region positioned directly adjacent to a P dopedsemiconductor region. Preferably for fabricating solar cells exhibitinggood efficiencies, P and N regions are in physical contact and have anabrupt interface without undoped semiconductor present between them. Theprocessed semiconductor material is subsequently patterned and etched todefine the physical dimensions of a printable a P-N junction. Subsequentprocessing via lift-off techniques generates a monolithic structurecomprising the printable P-N junction having a P doped layer directlyadjacent to an N doped semiconductor layer. The printable P-N junctionis then assembled onto a substrate using solution printing or drytransfer contact printing methods of the present invention. As shown inFIG. 34A, contacts (i.e. electrodes) on P and N doped semiconductorlayers may be defined by deposition on to the monolithic structure priorto lift-off processing of the printable P-N junction or by depositiononto the printable P-N junction after assembly on the substrate. In oneembodiment, contacts are defined using vapor deposition of one or moremetals.

FIG. 34B shows a schematic diagram of a solar cell device configurationgenerated by the fabrication pathway illustrated in FIG. 34A. A 5 micronthick P doped semiconductor layer having boron dopant is provided indirect contact with two N doped semiconductor layers having phosphorousdopant. Contacts are provided directly on the N doped layers and on twoenriched P doped layers in contact with the P doped semiconductor layerforming the P-N junction. Introduction of phosphorous and boron dopedcontact regions overcomes the contact resistance of the system. FIG. 34Cshows a plot of current verse bias showing the photodiode responseobserved upon illumination of a solar cell device having theconfiguration shown in FIG. 34B. As shown in FIG. 34C current isgenerated when the solar cell is illuminated and provided with apositive bias.

FIG. 35A provides a process flow diagram schematically illustratingprocessing steps in an alternative fabrication pathway for making asolar cell comprising independently printable P and N dopedsemiconductor layers. As shown in FIG. 35A, a high quality semiconductormaterial, such as a single crystalline silicon wafer, is provided andprocessed in a manner generating discrete N doped and P dopedsemiconductor regions. The processed semiconductor material issubsequently patterned and etched to define the physical dimensions ofseparate P doped and N doped layers. Subsequent processing via lift-offtechniques generates a separately printable P doped semiconductor layerand/or a separately printable N doped semiconductor layer. A P-Njunction is then assembled by printing a first doped semiconductorelement (either P or N doped) onto a second doped semiconductor elementhaving a different composition such that it is in contact with the firstdoped element. In one embodiment, the P-N junction is assembled byprinting both P and N doped semiconductor layers, for example byprinting a first doped semiconductor layer onto a substrate andsubsequently printing a second doped semiconductor layer onto the firstdoped semiconductor layer. Alternatively, the PN junction may beassembled by printing a first doped semiconductor layer onto a substratecomprising a second doped semiconductor layer. Any orientation of P andN doped layers providing a good interface between these elements isusable in the present invention including, but not limited to, aorientation in which the first doped semiconductor element is contactedto the top of the second doped semiconductor element.

Joining N and P doped printable semiconductor elements may beaccomplished via wafer bonding techniques well known in the art (See,e.g. “Materials Science and Engineering R” Jan Haisma and G.A.C.M.Spierings, 37 pp 1-60 (2002)). Optionally, P and N doped semiconductorlayers are treated prior to, during or after printing to strip away anyouter insulating layers on them, such as outer oxide layers, which caninterfere with establishing a P-N junction having a high qualityinterface between P-N doped layers. Optionally, in some embodiments anywater present on the doped semiconductor surfaces to be joined iseliminated, for example by heating, prior to contacting these elementsto enhance the quality of the interface in the P-N junction. Assembly ofthe first and second doped semiconductor elements may be carried outusing solution printing or dry transfer contact printing methods of thepresent invention. Optionally, the fabrication pathway of this aspect ofthe present invention may further comprise the step of annealing the P-Njunction to establish a good interface between P and N dopedsemiconductor layers. Annealing is preferably carried out attemperatures low enough so as not to significantly damage the substratesupporting the P-N junction, for example at temperatures less than about200 degrees Celsius for P-N junctions assembled on plastic substrates.Alternatively, the P-N Junction may be annealed in processing stepsseparate from the substrate. In this embodiment, the annealed P-Njunction is allowed to cool and subsequently assembled onto thesubstrate via solution printing or dry transfer contact printingmethods. As shown in FIG. 35A, contacts (i.e. electrodes) on P and Ndoped semiconductor layers may be defined by deposition on to individualdoped semiconductor layers prior to lift-off processing or by depositiononto the printable P-N junction after assembly on the substrate. In oneembodiment, contacts are defined using vapor deposition of one or moremetals.

FIG. 35B shows a schematic diagram of a solar cell device generated byprinting a N doped semiconductor layer on top of a P doped semiconductorlayer of a silicon wafer. The composite structure was annealed to atemperature of about 1000 degrees Celsius to generate a P-N junctionhaving a high quality interface between N and P doped semiconductorlayers. Electrical contacts were provided directly on top of each dopedsemiconductor layer via vapor deposition of aluminum layers. FIG. 35Cshows a SEM image of a top view of the solar cell schematically depictedin FIG. 35B. The SEM image shows the N doped semiconductor layerpositioned on top of the P doped semiconductor layer and also shows thealuminum contacts on top of each doped semiconductor layer. FIG. 35Dprovides a plot of current verse bias demonstrating the photodioderesponse of the solar cell shown in FIG. 35C. As shown in FIG. 35Dcurrent is generated when the solar cell is illuminated and providedwith a positive bias. FIG. 35E provides a plot of photocurrent as afunction of time observed upon illumination of the solar cell shown inFIG. 35C with different intensities of light.

The physical dimensions of printable heterogeneous semiconductorelements, such as printable doped semiconductor elements and printableP-N junctions, useful in solar cells of the present invention depend ona number of variables. First, the thickness must be large enough thatappreciable fractions of the incident photons per area are absorbed bythe P-N junction. Therefore, the thicknesses of P and N doped layerswill depend, at least in part, on the optical properties of theunderlying semiconductor material, such as its absorption coefficient.For some useful applications, thicknesses of printable silicon elementsrange from about 20 microns to about 100 microns and thicknesses ofgallium arsenide elements range from about 1 micron to about 100microns. Second, in some device applications the thickness of theprintable elements must be small enough that they exhibit a usefuldegree of flexible for a particular device application. Use of thin(<100 microns) elements provides flexibility, even for brittle materialssuch as single crystalline semiconductors, and also lowers fabricationcosts by requiring less raw materials. Third, the surface area of theprintable elements should be large so as to capture a significant numberof incident photons.

Dopants can be introduced into the semiconductor material by any processcapable of providing well defined spatial distributions of high qualitydoped semiconductor materials, including methods using spin-on dopants(e.g., see Example 8). Exemplary methods of introducing dopant intosemiconductor materials prove control in the spatial distribution ofdopants in one, two or three dimensions (i.e. depth of implantation andarea of a semiconductor layer implanted with dopant). A significantadvantage of the fabrication pathways shown in FIGS. 34A and 35A is thatdopant implantation and activation may be separately carried out underclean room conditions and at high temperatures. Subsequent fabricationand assembly of printable doped semiconductor elements and/or P-Njunctions, however, may be carried out at lower temperatures and innon-clean room conditions, thereby allowing high throughput fabricationof solar cells on a variety of substrate materials.

Example 9: Fabrication of Stretchable Circuits and Electronic Devices

The present invention provides stretchable electrical circuits, devicesand device arrays capable of good performance when stretched, flexed ordeformed. Similar to the stretchable semiconductor elements described inExample 2, the present invention provides stretchable circuits andelectronic devices comprising a flexible substrate having a supportingsurface in contact with a device, device array or circuit having acurved internal surface, such as a curved internal surface exhibiting awave structure. In this structural arrangement, at least a portion ofthe curved internal surface of the device, device array or circuitstructure is bonded to the supporting surface of the flexible substrate.In contrast to the stretchable semiconductors in Example 2, however, thedevice, device array or circuit of this aspect of the present inventionis a multicomponent element comprising a plurality of integrated devicecomponents, such as semiconductors, dielectrics, electrodes, dopedsemiconductors and conductors. In an exemplary embodiment, flexiblecircuits, devices and device arrays having a net thickness less thanabout 10 microns comprise a plurality of integrated device components atleast a portion of which have a periodic wave curved structure.

In a useful embodiment of the present invention, a free standingelectrical circuit or device comprising a plurality of interconnectedcomponents is provided. An internal surface of the electrical circuit ordevice is contacted and at least partially bonded to a prestrainedelastic substrate in an expanded state. Prestraining can be achieved byany means known in the art including, but not limited to, roll pressingand/or prebending the elastic substrate, and the elastic substrate maybe prestrained by expansion along a single axis or by expansion along aplurality of axes. Bonding may be achieved directly by covalent bondingor van der Waals forces between at least a portion of the internalsurface of the electrical circuit or device and the prestrained elasticsubstrate, or by using adhesive or an intermediate bonding layer. Afterbinding the prestrained elastic substrate and the electrical circuit ordevice, the elastic substrate is allowed to relax at least partially toa relaxed state, which bends the internal surface of the printablesemiconductor structure. Bending of the internal surface of theelectrical circuit or device generates a curved internal surface whichin some useful embodiments has a periodic or aperiodic waveconfiguration. The present invention includes embodiments wherein allthe components comprising the electrical device or circuit are presentin a periodic or aperiodic wave configuration.

Periodic or aperiodic wave configurations of stretchable electricalcircuits, devices and device arrays allow them to conform to stretch orbent configurations without generating large strains on individualcomponents of the circuits or devices. This aspect of the presentinvention provides useful electrical behavior of stretchable electricalcircuits, devices and device arrays when present in bent, stretched ordeformed states. The period of periodic wave configurations formed bythe present methods may vary with (i) the net thickness of thecollection of integrated components comprising the circuit or device and(ii) the mechanical properties, such as Young's modulus and flexuralrigidity, of the materials comprising integrated device components.

FIG. 36A shows a process flow diagram illustrating an exemplary methodof making an array of stretchable thin film transistors. As shown inFIG. 36A, an array of free standing printable thin film transistors isprovided using the techniques of the present invention. The array ofthin film transistors is transferred to a PDMS substrate via drytransfer contact printing methods in a manner which exposes internalsurfaces of the transistors. The exposed internal surfaces are nextcontacted with a room temperature cured prestrained PDMS layer presentin an expanded state. Subsequent full curing of the prestrained PDMSlayer bonds the internal surfaces of the transistors to the prestrainedPDMS layer. The prestrained PDMS layer is allowed to cool and assume anat least partially relaxed state. Relaxation of PDMS layers introduces aperiodic wave structure to the transistors in the array, thereby makingthem stretchable. The inset in FIG. 36A provides an atomic forcemicrograph of a array of stretchable thin film transistors made by thepresent methods. The atomic force micrograph shows the periodic wavestructure that provides for good electrical performance in stretched ordeformed states.

FIG. 36B shows provides optical micrographs of an array of stretchablethin film transistors in relaxed and stretched configurations.Stretching the array in a manner generating a net strain of about 20% onthe array did not fracture or damage the thin film transistors. Thetransition from a relax configuration to a strain configuration wasobserved to be a reversible process. FIG. 36B also provides a plot ofdrain current verse drain voltage for several potentials applied to thegate electrode showing that the stretchable thin film transistorsexhibit good performance in both relaxed and stretched configurations.

Example 10: Large Area, Selective Transfer of Printable MicrostructuredSilicon (μs-Si): A Printing-Based Approach to High Performance Thin FilmTransistors Supported on Flexible Substrates

The methods, devices and device components of the present inventionprovide a new printing-based fabrication platform for making highperformance integrated microelectronic devices and device arrays.Advantages of the present approach to macroelectronic andmicroelectronic technologies over conventional processing methodsinclude compatibility with a wide range of substrate materials, physicaldimensions and surface morphologies. In addition, the presentprinting-based approach enables a low cost, high efficiency fabricationpathway for making integrated microelectronic devices and device arrayson large areas of substrates that is compatible with pre-existing highthroughput printing instrumentation and techniques.

The advanced information technologies that shape the structure of modernsociety depend critically on the use of microelectronic devices, onesthat involve ever increasing higher densities of integration. From theinitial circuits (ICs) of the late 1950's, ones that incorporated fewerthan 4 transistors, current state of the art ICs now integrate millionstransistors in an essentially equivalent sized package. There has beenan increased interest, however, in developing new device form factors,ones in which the capabilities of semiconductor devices are embedded instructure involving either large area and/or flexible materials supportsusing fabrication method that serve to in an attempt to decrease costswhile maintaining high device performance levels. Such devicestechnologies could find wide application as active matrix pixel displaydrivers and components of RF identification tags. Recent reports detailthe use of solution processing methods to construct models of suchcircuits, notably ones based on semiconductor nanowires (NWs) ornetworked nanotubes. Although functional devices prepared in this wayare promising, they are generally characterized by significantly lowerlevels of device performance compared to conventional high temperaturesemiconductor processing approaches. For example, field effectivemobilites ranging from ˜2 cm²/Vs and ˜40 cm²/Vs are reported for thinfilm transistors (TFTs) prepared using solution processing methods.

In one aspect, the present invention provides a “top down” fabricationstrategy using microstructured single-crystalline silicon (μs-Si)ribbons harvested from silicon-on insulator wafers for use in ultra-highperformance TFTs. This fabrication technique is compatible with respectto a range of useful semiconductor materials, and has been successfullyadapted to other industrially useful semiconductor materials thatinclude GaN, InP and GaAs.

In this example we demonstrate a number of important processing stepsuseful in the implementation of this technology, including fabricationmethods which allow the selective transfer and accurate registration ofsilicon ribbons across large substrate areas, and versatile printingprocedures applicable to both rigid (i.e. glass) and flexible plasticsubstrates. We specifically report here two methods that can be used toselectively remove μs-Si from an SOI wafer and subsequently transferthem in patterned forms onto a plastic substrate. The processes, forconvenience referred to have as Method I (FIG. 37A) and Method II (FIG.37B), use different mechanisms of adhesive bonding to affect theprinting-based pattern transfer of the μs-Si. Method I exploits physicalbonding between a molded Sylgard 3600 poly(dimethylsiloxane) (PDMS)stamp (a new experimental, high modulus PDMS product provided by the DowCorning Corp.) and μs-Si objects. Method II uses a recently developedmasterless soft-lithography technique to chemically bond the μs-Si to aPDMS coated substrate.

FIG. 37A provides a schematic diagram showing a processing method of thepresent invention (Method I) for patterning μs-Si elements onto aplastic substrate. In the present example, the plastic substratecomprised a poly(ethyleneterepthalate) (PET) sheet. A peanut shapedphotoresist pattern is developed on top of a SOI substrate usingstandard photolithography techniques. Plasma etching, followed by resiststripping, yields μs-Si “peanuts” that are supported on top of a buriedoxide layer. The sample is then etched incompletely using HF to giveundercut peanuts held only by a residual oxide layer present at thedumbbell ends of the μs-Si. The SOI wafer is then laminated with a hard3600 PDMS stamp molded with features corresponding to the latent imageof the desired pattern transfer. The raised features of the stampcorrespond to regions where μs-Si is removed selectively from the SOIsurface due to strong autoadhesion to the PDMS. The stamp, after pealingit away from the SOI wafer, is then placed in contact with apoly(ethyleneterepthalate) (PET) sheet coated with polyurethane (PU)that had been partially cured using a UV lamp. A bar coating techniqueis used to deposit the PU adhesion level to ensure a uniform coatingthickness over the large area of the (600 cm²) plastic substrate. Theμs-Si on the stamp is then placed in contact with the PU coated side ofthe plastic sheet, a second UV/Ozone exposure is then preformed from thePET side of the sandwich to fully cure the PU and enhance its bonding tothe μs-Si. Pealing the stamp from the plastic substrate results in thedetachment of the microstructured silicon from the PDMS, thus completingthe transfer to the PU coated substrate.

FIG. 37B provides a schematic diagram illustrating an alternativeprocessing method of the present invention (Method II) for patterningμs-Si elements onto a plastic substrate. In the present example, theplastic substrate comprised a poly(ethyleneterepthalate) (PET) sheet.This recently reported Decal Transfer Lithography (DTL) techniqueeffects the pattern transfer using a flat, unmolded PDMS slab that isphotochemically treated to provide spatially modulated strengths ofadhesion. An UV/Ozone (UVO) treatment is patterned across the surface ofa slab of conventional Sylgard 184 PDMS using a microreactor photomaskto pattern the UVO modification with high spatial resolution. Afterexposure, the photochemically modified PDMS coated PET is placed incontact with a peanut presenting SOI wafer and heated to 70° C. for 30minutes. The fabrication of the peanut shapes on the SOI wafer followedthe same procedures of Method I (see FIG. 37A), with the addition ofevaporating a thin film of SiO₂ (5 nm) onto the surface after the HFetching step. This layer facilitates strong chemical bonding to thePDMS. After heating, the PDMS is pealed from the SOI, giving a patternedtransfer of μs-Si to the UVO modified regions of the PDMS.

FIG. 38A shows the design of the so-called peanut shaped μs-Si objectsused in methods of the present invention. Inset optical image in FIG.38A shows the optimized HF etching condition where the buried oxideunder the channel is removed while a sacrificial SiO2 portion remains.The peanut shape is particularly beneficial because its ends areslightly wider than the body of the structure. Upon etching theunderlying oxide layer in an HF solution, the timing can be optimizedsuch that the oxide layer under the center is completely removed while asacrificial portion of SiO₂ still remains at either end (the dumbbellregion seen in the inset image of FIG. 38A). It is this residual SiO₂layer that holds the μs-Si in its original position. Without this oxidebridge layer, the order of the μs-Si created on the SOI wafer byphotolithography is susceptible to lost. FIG. 38B shows an example oflost of this order when the Si objects are overetched in HF solution. Asshown in FIG. 38B, Si objects to start to float in the HF solution whenthe sample was over etched in HF solution. When the μs-Si is removedfrom the SOI wafer by either Method I or II, fracture occurs at theedges of the sacrificial region.

FIGS. 38C, 38D, 38E and 38F shows a series of micrographs that depictsthe progression of each step of the μs-Si transfer as effected usingMethod I. FIG. 38C shows the μs-Si on the SOI wafer after optimizedundercut HF etching. FIG. 38D shows the SOI wafer after the PDMS stampremoved a portion of the μs-Si. As shown in FIG. 38D, the PDMS stampremoves a portion of the μs-Si, thereby leaving the neighboring regionsintact on the SOI. Since the unused microstructured silicon objects onthe SOI wafer are retained at their original positions, they can bepicked up by a stamp and transferred in subsequent printing steps (asdiscussed below). FIG. 38E shows μs-Si structures transferred onto thePDMS stamp. The missing center of each end of the μs-Si ribbons revealsthe pattern of the fracture occurs during the transfer of themicrostructured silicon from the SOI to PDMS stamp. FIG. 38F shows arepresentative result for a second transfer of the μs-Si (this time fromthe PDMS stamp to the PU coated plastic substrate) wherein the μs-Sithat adhered to the PU support on the plastic.

Multiple transfers are possible from a small PDMS stamp to a largerplastic surface. FIGS. 39A and 39B provide optical images of theselective transfer of the μs-Si onto PU/PET sheet by 3600 PDMS stamp. Asshown in FIG. 39A, a large area (15×15 cm) transfer where the μs-Si wassparsely transferred onto a plastic substrate by multiple transfersusing a 8×8 cm stamp. Each pixel in the image is of the sameconfiguration as that shown in FIG. 38F and follows the same protocoldescribed for FIGS. 38C-38E. The inset of FIG. 39B shows a more complexmolded form, a “DARPA macroE” lettering composed of peanut μs-Si objectssmaller in size than those highlighted in FIGS. 38C-38E. The highpattern fidelity of the transfer is illustrated by the qualities of theobjects defining the letter “A” (circle of inset image) as shown in FIG.39B. These data demonstrate that only those areas directly touched bythe stamp ultimately transfer to the plastic substrate. We note thatthis transfer is more difficult using conventional Sylgard 184 PDMS fortwo reasons. First, the Sylgard 184 sags when the separation distancesbetween features exceeds twenty times the feature height. The examplesshown here embrace such design rules and thus precludes high-fidelitytransfers using the lower modulus polymer. Second, we also found thatthe Sylgard 184 sometimes does not have enough adhesive force to pickupevery μs-Si peanut from the SOI wafer and defects are observed in someapplications using stamps prepared form this polymer. The 3600 PDMS fromDow Corning does not sag appreciably even at an aspect ratios of 1:200and, perhaps more importantly, its adhesion to the μs-Si objects isstronger than is that of the 184 PDMS.

An example of a μs-Si transfer carried out using Method II is shown inFIGS. 39C and 29D. FIG. 39C is an optical micrograph of a section of aSylgard 184 coated PET substrate to which the μs-Si has been chemicallybonded and subsequently transferred. A higher magnification image of theμs-Si transferred in this way is shown in FIG. 39D. It should be notedthat the dimensions of the peanuts used in this demonstration arerelatively small with ribbon widths of 25 μm. We found, interestingly,that these smaller features have a different fracture point when theyare removed from the SOI wafer. In the blowup of FIG. 39D, one alsonotes that the PDMS surface is also no longer flat. The reason for thisis because of the fact that sections of the PDMS are in factreciprocally transferred to the SOI, being ripped out of the bulk incontacting regions activated by the patterned UVO treatment, regionswhere the PDMS sagged and touched the wafer surface between the peanuts.

FIG. 40A illustrates an exemplary device geometry of a device fabricatedusing the peanut shaped μs-Si based on a transfer using Method I. Toconstruct these devices an Indium-Tin-Oxide (ITO) coated PET sheet isused as the substrate. The ITO served as the gate electrode and dilutedSU-8 5 (measured capacitance=5.77 nF/cm²) is employed as a gatedielectric. FIG. 40B provides I-V curves of μs-Si TFTs at a range ofgate voltage (Vg=−2.5 V to 20 V). As shown in FIG. 40B, these plasticsupported, peanut shaped μs-Si TFTs show an accumulation mode n-channeltransistor behavior. The channel length of the device, as shown in theinset image of FIG. 40C, is 100 μm and the width of the device is 400μm. FIG. 40C shows the transfer characteristics, measured at a constantsource-drain voltage (Vsd=1V), indicated the effective mobility was 173cm²/Vs. The inset in FIG. 40C shows an optical micrograph of actualdevice of the present invention. The transfer characteristics indicatedthat the threshold voltage (Vth) is ˜2.5 V with an effective mobilitywas 173 cm²/Vs. These values are consistent with the performancecharacteristics expected for a 100 nm thick bottom gate structure ofthis type.

The selective transfer methods described in this example provide anefficient route for transferring microstructured silicon from a SOIwafer to a flexible, macroelectronic system. Using these techniques, andin contrast with conventional solution casting methods, themicrostructured silicon objects can be transferred from an SOI motherwafer with precise registration and utilized in ways that minimizewaste. The mechanical properties of the new 3600 PDMS investigated inthis work demonstrates that it has a number of important advantages ascompared to the commercial Sylgard 184 PDMS resin, notably itsdimensional stability and higher surface adhesion properties. Theprinting techniques also proved to be compatible with the constructionof macroelectronic systems that incorporate high performance μs-Si thinfilm transistors.

Experimental

Method I

The fabrication of the μs-Si objects was carried out using a commercialSOI wafer (SOITEC, p-type, top Si thickness=100 nm,resistivity=13.5-22.5 ohm-cm, 145 nm buried oxide layer).Photolithography (Shipley 1805 resist) was used to pattern the SOI waferinto the desired peanut-shaped geometry (mid-section length: 200 μm,width: 25 μm, diameter of peanut: 50 μm). Dry etching (Plasmatherm RIEsystem, SF6 flow, 40 sccm, 50 mTorr, RF power=100 W, 45 sec) was thenused to remove the exposed silicon. The underlying SiO₂ was then etchedfor 80 seconds in an HF (49%) solution. For the 3600 PDMS stamp ofMethod I, a specialty PDMS (Dow Corning, 3600, elastic modulus=8 MPa)and Sylgard 184 (Dow corning, elastic modulus=1.8 MPa) was mixed in aone to one ratio and cured using standard soft-lithographic patterningmethods A UV source (ozone active mercury lamp, 173 μW/cm²) was used tocure the PU thin film adhesion layer (Norland optical adhesive, No. 73).These latter films were coated onto a PET substrate (180 μm inthickness, Mylar film, Southwall technologies) using a bar coatingprocedure (Meyer bar, RD specialties).

Method II

For Method II, the sizes of the peanut shapes used were smaller than theones used in Method I (mid-section length: 10 μm, width: 2 μm, diameterof ends: 5 μm). A similar fabrication protocol was used to produce thesestructures with the exception that the RIE etching time was reduced to25 seconds (to minimize sidewall etching) and the buried oxide layer wasetched for 30 seconds in a concentrated (49%) HF solution. After thelatter etching step, the sample was rinsed in a water bath and dried inan oven at 70° C. for 5 minutes. A 50 Å SiO₂ layer was then evaporatedon top of the sample (Temescal FC-1800 Electron Beam Evaporator). Tobind a thin layer of PDMS onto the PET substrate, a layer of PU wasfirst cast by spinning onto the PET at 1000 rpm for 30 seconds andexposed to UVO (173 μW/cm²) for 4 minutes. A film of PDMS was thenspuncast at 1000 rpm for 30 seconds onto the PU a cured thermally at 65°C. for three hours.

The selective area soft lithographic patterning procedure comprisedplacing the unpatterened PDMS side of the coated PET substrate incontact with the patterned side of the UVO photomask. The fabrication ofthis microreactor mask followed procedures described by Childs et. al.The pattern consisted of two interlocking rectangular arrays (1.2×0.6mm). The PDMS was then irradiated through the UVO photomask for 3minutes at a distance of ˜3 cm from a mercury bulb (UVOCS T10×10/OES).After exposure, the PDMS stamp was pealed away from the UVO photomask,and the exposed PDMS face was placed into contact with thepeanut-bearing SOI wafer. After heating at 70° C. for 30 minutes,tweezers were used to slowly peal the PDMS away, removing segments ofthe μs-Si in registry with the areas of irradiation.

Device Fabrication

SU-8 5 with 66% (v) SU-8 2000 thinner was spun onto the ITO side of acoated PET sample at 3000 rpm for 30 seconds. The SU-8 epoxy was thenprecured at 60° C. on a hot plate for ˜1 minute. The PDMS stamp (MethodI) with the μs-Si on its surface was then brought into contact with theepoxy layer for 30 seconds and peeled back to transfer the μs-Si to theepoxy. The SU-8 dielectric was then fully cured at 115° C. for 2minutes, exposed to UV for 10 seconds, and postbaked at 115° C. for 2minutes. Metal for titanium contacts (40 nm) was then added by e-beamevaporation, with the source-drain area patterned using standardphotolithographic methods in conjunction with etching using a 1% HFsolution.

Example 11: Bendable GaAs Metal-Semiconductor Field Effect TransistorsFormed with Printed GaAs Wire Arrays on Plastic Substrates

The fabrication methods of the present invention are versatility withrespect to the materials that can be assembled and integrated intouseful functional devices and device components. Particularly, thepresent methods are applicable to fabrication of semiconductor basedmicroelectronic and macroelectronic devices using a range of highquality semiconductor materials, including non-silicon materials. Todemonstrate this capability of the present methods bendablemetal-semiconductor field-effect-transistors (MESFETs) having GaAsmicrowires were fabricated by present methods and evaluated with respectto their electrical and mechanical attributes.

Field effect transistors formed with high quality, single crystallinesemiconductor nano- and microstructures on large area, mechanicallyflexible plastic substrates are of great interest for a wide range ofapplications in displays, sensors, medical devices and other systems. Anumber of approaches have been demonstrated to transfer high qualitysemiconductor materials (e.g., Si nanowires, microribbons, platelets,etc.) onto plastic substrates for mechanically flexiblemetal-oxide-semiconductor field-effect-transistors (MOSFETs). Themethods of the present invention are useful for fabricating bendablemetal-semiconductor field-effect-transistors (MESFETs) on plasticsubstrates using GaAs microwires (a class of material that we refer toas microstructured GaAs, or μs-GaAs) that have integrated ohmicsource/drain contacts. In these methods, high quality bulk GaAs wafersprovide the starting material for ‘top down’ fabrication procedures toform the micro/nanowires. In addition, transfer printing techniquesusing elastomeric stamps integrate well ordered arrays of these wireswith plastic substrates. Electrical and mechanical measurements ofMESFETs formed in this way demonstrate that good performance andexcellent bendability is achievable using the present methods.

FIG. 41 provides a schematic illustration of the major steps forfabricating, on flexible plastic substrates (poly(ethyleneterephthalate) (PET)), MESFETs that use arrays of single crystallineGaAs wires with expitaxial n-type channel layers, and integrated ohmiccontacts of AuGe/Ni/Au. A (100) semi-insulating GaAs (SI—GaAs) waferwith an epitaxial Si-doped n-type GaAs layer (concentration of4.0×10¹⁷/cm³, IQE Inc., Bethlehem, Pa.) provides the source material forgenerating the microwires. Photolithography and metallization viaelectron-beam (and/or thermal) evaporation generates arrays of narrowmetal stripes (with width of 2 μm and spacing of 13 μm) comprisingconventional multilayer stacks, i.e., AuGe (120 nm)/Ni (20 nm)/Au (120nm) for ohmic contacts. Annealing the wafer at elevated temperature(i.e., 450° C. for 1 min) in a quartz tube with flowing N₂ forms ohmiccontacts to the n-GaAs.

Defining the metal stripes along the (0 1 1) crystalline orientation ofGaAs enables microwires (with integrated ohmic contacts) to be generatedusing a top down fabrication approach. As shown in processing step i inFIG. 41, a pattern of photoresist is defined on top of the metal stripes(3 μm widths); the openings between these lines lie between adjacentmetal stripes. These openings allow etchant (H₃PO₄ (85 wt %):H₂O₂ (30 wt%):H₂O=1:13:12 in volume) to diffuse to the GaAs surface to etch GaAsanisotropically. The photoresist protects the interface between ohmicstripes and GaAs from exposure. The anisotropic etching generatesreverse mesas and undercuts along the surface of GaAs, resulting in theformation of GaAs wires with triangular cross section and narrow widthreleased from mother wafer. The undercut yields GaAs wires with widthsdown to micrometer and/or nanometer length scales by controlling thegeometry of the resist and the etching time. Each wire has two ohmicstripes separated by a gap that defines the channel length of theresultant MESFET. As shown in processing step ii in FIG. 41, contactinga flat, elastomeric stamp of poly(dimethylsiloxane) (PDMS) to thephotoresist coated GaAs wires forms a van der Waals bond between thehydrophobic surfaces of the PDMS and the photoresist. As shown inprocessing step iii in FIG. 41, this interaction enables removal of allof the GaAs wires from the wafer to the surface of the PDMS when thestamp is peeled back from the mother wafer. This transfer processpreserves the lithographically defined spatial organization (i.e.aligned arrays) of the wires. The PDMS stamp with GaAs wires is thenlaminated against a PET sheet covered with a thin layer of liquidpolyurethane (PU, NEA 121, Norland Products Inc., Cranbury, N.J.), akind of photocurable polymer.

As shown in processing step iv in FIG. 41, curing the PU, peeling offthe PDMS stamp and removing the photoresist by O₂ reactive ion etching(RIE, Uniaxis 790, Plasma-Therm Reaction Ion Etching System) leaves theordered GaAs wires with exposed ohmic stripes embedded on the surface ofthe PU/PET substrate. In the transfer printing process, the photoresistserves not only as an adhesive layer but also as a protective film toprevent the surfaces of the GaAs wires and ohmic contacts from beingcontaminated. As shown in processing step v in FIG. 41, furtherlithographic processing on the PU/PET substrate defines electrodes (250nm Au) that connect the ohmic stripes to form the source and drain, andfor gate electrodes (Ti (150 nm)/Au (150 nm)). The resultant arrays ofMESFETs are mechanically flexible due to the bendability of PU/PET sheet(thickness of ˜200 μm) and the GaAs wires (widths and thicknesses lessthan 5 μm).

FIG. 42A presents a schematic showing a cross section view of thegeometry of a GaAs wire based MESFET on a plastic substrate (PU/PET).The source/drain electrodes form ohmic contacts to the n-GaAs layer. Thegate electrode forms a Schottky contact to this layer. The stronginteraction between the cured PU and the side walls of the GaAs wiresbonds the wires to the PU/PET substrate. In this geometry and with theprocessing approach described previously, the active n-GaAs layer (i.e.,the transistor channel) never contacts any polymeric materials otherthan photoresist. The Ti/Au gate electrode forms a Schottky contact withthe n-GaAs surface; the barrier allows one to apply a relativelynegative voltage (i.e., <0.5 V) to modulate the flow of current betweenthe source and drain electrodes, as in a conventional MESFET. FIG. 42Bshows a representative image of two GaAs wire based MESFETs on plasticeach of which uses an array of ten GaAs wires, fabricated according tothe process flow diagram of FIG. 41. The wires have well-alignedorientation and uniform widths of ˜1.8 μm. Au pads with widths of 150 μmand lengths of 250 μm connect the ohmic stripes on ten GaAs wires toform source and drain electrodes for each individual MESFET. A Ti/Austripe with width of 15 μm and deposited in the 50-μm gap (transistorchannel) between source and drain electrodes provides the gateelectrode. These stripes connect to a larger metal pad for probing. Thecontrast difference between the metal on the wires and that on theplastic is likely due to surface roughness on the PU generated duringRIE etching of the photoresist. FIG. 42C shows the image of a 2 cm×2 cmPET sheet with hundreds of transistors, clearly demonstrating itsflexibility. Multiple printing steps and/or wire fabrication runs may beused to generate large numbers of wires patterned over large areas onplastic substrates. Various parameters, such as the widths of the GaAswires, the widths of the source/drain electrodes, the channel and gatelengths are adjusted easily to yield MESFETs with a range of desiredoutput characteristics.

The DC performance of the transistors was characterized to evaluatetheir electrical and mechanical properties. FIGS. 43A, 43B and 43Cpresent results from a GaAs MESFET with a channel length of 50 μm, agate length of 15 μm, similar to the one shown in FIG. 42B. FIG. 43Ashows the current-voltage (between drain and source electrodes) curvesat gate voltages between 0.5 to ˜2.0 V with steps of 0.5 V. TheI_(DS)-V_(DS) characteristics are comparable to conventional wafer basedMESFETs built with n-type GaAs layer and standard techniques, i.e.,I_(DS) saturates in the regions of high V_(DS) and I_(DS) decreases withdecrease of gate voltage. In the linear region, the channel resistanceat V_(GS)=0 V is R_(channel)=6.4 kΩ. FIG. 43B shows the transfercharacteristics (i.e., I_(DS) vs. V_(GS)) of a GaAs MESFET of thepresent invention measured at different V_(DS). All of the curves haveminima at the same gate voltage, i.e., ˜2.65 V. The drop of I_(DS) athigh positive gate voltages is due to the leakage current from gate tosource that develops through the Schottky contact in this regime. FIG.43C shows the transfer curve at V_(DS)=4 V, plotted as (I_(DS))^(1/2)vs. V_(GS), clearly showing a linear relationship as expected for aMESFET. The pinch-off voltage and transconductance at I_(DS)=0.19 mA andV_(DS)=4 V are V_(p)=2.65 V and g_(m0)=168 μS, respectively. Thesecharacteristics indicate that the transistors fabricated on PETsubstrates resemble the behavior of typical GaAs MESFETs fabricated onwafers by traditional approach.

Mechanical flexibility represents an important parameter of devices onplastic substrates for many of the target applications that are beingconsidered. We tested the transistors by bending the supporting PETsheet. FIGS. 44A and 44B show gate-modulated current-voltagecharacteristics of a GaAs wire based MESFET on a flexible PET substrate(A) before bending; (B) after bending to a bend radius of 8.4 mm. Thesefigures compare the performance of a transistor before and after thesubstrate was bent to a radius of 8.4 mm, i.e., corresponding surfacestain of 1.2% (tensile in this case) for the 200 μm thick substrate. Theresults indicate that the transistor can withstand these high strainswithout failure. In fact, the saturated current at V_(GS)=0 V, increasesby ˜20% in this case. FIG. 44C shows the gate-modulated current-voltagecharacteristics of the GaAs wire based MESFET after relaxing the bentsubstrate to its flat, unbent state. A comparison of FIG. 44C and FIG.44A indicates that after releasing the strain, i.e., such that thesubstrate becomes flat again, the transistor recovers the performance ofits original state. FIG. 44D shows the variation of I_(DS) at V_(DS)=4 Vand V_(GS)=0 V in 3 cycles in terms of bending (with different surfacestrains)/unbending, indicating that these MESFETs survive multiplebending cycles that cause the tensile strain at the device to varybetween 0% and 1.2%, without significant change of their performance(<20%). The systematic changes observed with strain may be related tothe fact that mechanical strain causes displacement of crystallinelattice of GaAs wires and their distribution of energy levels.

This example describes an approach that involves (i) generation of ohmiccontacts by high temperature annealing on GaAs wafers, (ii) productionof GaAs microwires with these integrated ohmic contacts by anisotropicchemical etching, (iii) dry transfer printing of these wires ontoplastic substrates with an elastomeric stamp, and (iv) fabrication ofhigh quality MESFETs by low temperature processing of these wires onplastics, to yield the flexible GaAs MESFETs on plastic substrates. Theintrinsic properties of GaAs (e.g. high mobilities), the ability to makethe MESFETs with short gate lengths and the straightforward paths forintegrating these devices into complex circuits (potentially with othertransistors built using similar approaches but with othersemiconductors) indicate a use for achieving high frequency response foradvanced communication, space and other systems. These advantages aswell as the remarkably good mechanical flexibility of these devices makeGaAs wire MESFETs interesting for flexible macroelectronic systems.

In summary, Micro/nanowires of GaAs with integrated ohmic contacts havebeen prepared from bulk wafers by metal deposition and patterning, hightemperature annealing and anisotropic chemical etching. These wiresprovide a unique type of material for high performance devices that canbe built directly on a wide range of unusual device substrates, such asplastic or paper. In particular, transfer printing organized arrays ofthese wires at low temperatures onto plastic substrates yield highquality, bendable metal-semiconductor field effect transistors(MESFETs). Electrical and mechanical characterization of devices onpoly(ethylene terephthalate) illustrates the level performance that canbe achieved. These results indicate promise for this approach to highspeed flexible circuits for emerging applications in consumer andmilitary electronic systems.

Example 12: Device Configurations Using Printable Semiconductor Elements

FIG. 45 provides a schematic diagram illustrating an exemplary deviceconfiguration of the present invention for a P type bottom gate thinfilm transistor on a plastic substrate. As shown in FIG. 45, the P typebottom gate thin film transistor comprises a silicon printablesemiconductor element with doped contact regions, an indium tin oxidebottom gate electrode, an epoxy dielectric layer and source and drainelectrodes. The plastic substrate is a poly(ethylene terephthalate)(PET) sheet. Also provided in FIG. 45 are current-voltagecharacteristics typical of these devices for a range of gate voltages.

FIG. 46 provides a schematic diagram illustrating an exemplary deviceconfiguration of the present invention for a complementary logic gate ona plastic substrate. As shown in FIG. 46, the complementary logic gatecomprises a P-type thin film transistor and a N-type thin filmtransistor, each having printable semiconductor elements and provided ona poly(ethylene terephthalate) (PET) sheet.

FIG. 47 provides a schematic diagram illustrating an exemplary deviceconfiguration of the present invention for a top gate thin filmtransistor on a plastic substrate. As shown in FIG. 45, the top gatethin film transistor comprises a silicon printable semiconductor elementwith doped contact regions, a SiO₂ dielectric layer and gate, source anddrain electrodes. The plastic substrate is a poly(ethyleneterephthalate) (PET) sheet having a thin epoxy layer to facilitatetransfer and assembly of the thin film transistors and componentsthereof. Also provided in FIG. 47 are current-voltage characteristicstypical of these devices for a range of gate voltages.

We claim:
 1. A wafer of printable elements, the wafer comprising: asubstrate; and a plurality of printable elements native to thesubstrate, wherein each of the printable elements is at least partiallyreleased from the substrate and the printable elements are eachconnected to the substrate by one or more alignment maintainingelements; wherein each of the printable elements is at least partiallyundercut from the substrate.
 2. The wafer of claim 1, wherein each ofthe printable elements is disposed on the substrate.
 3. The wafer ofclaim 1, wherein each of the printable elements is mechanicallyconnected to the substrate only by one or more alignment maintainingelements.
 4. The wafer of claim 1, wherein the one or more alignmentmaintaining elements consist of one alignment maintaining element. 5.The wafer of claim 1, wherein each of the one or more alignmentmaintaining elements is laterally connected between the printableelement and the substrate in a direction parallel to a surface of thesubstrate on which the plurality of printable elements is disposed. 6.The wafer of claim 1, wherein the substrate comprises an inorganic,crystalline semiconductor material.
 7. The wafer of claim 1, wherein thesubstrate comprises an anisotropically etchable material.
 8. The waferof claim 1, wherein the plurality of printable elements comprises asemiconductor material and the substrate comprises the semiconductormaterial.
 9. A wafer of printable elements, the wafer comprising: asubstrate; and a plurality of printable elements disposed on thesubstrate, wherein each of the printable elements is at least partiallyreleased from the substrate, the printable elements are connected to thesubstrate by one or more alignment maintaining elements; and each of theprintable elements is at least partially undercut from the substrate.10. The wafer of claim 9, wherein the printable elements are native tothe substrate.
 11. The wafer of claim 9, wherein the substrate comprisesa semiconductor material.
 12. The wafer of claim 9, wherein theplurality of printable elements comprises a semiconductor material andthe substrate comprises the semiconductor material.
 13. The wafer ofclaim 9, wherein the printable elements are mechanically connected tothe substrate only by one or more alignment maintaining elements. 14.The wafer of claim 9, wherein the substrate comprises an anisotropicallyetchable material.
 15. A method of making a wafer of printable elements,comprising: providing a substrate; forming a plurality of printableelements that are native to the substrate; providing a stamp; contactingone or more printable elements of the plurality of printable elementswith the stamp; at least partially releasing each of the printableelements from the substrate so that the printable elements are eachconnected to the substrate by one or more alignment maintaining element;and removing the stamp from the substrate thereby fracturing the one ormore alignment maintaining elements connected to the contacted one ormore printable elements.
 16. A wafer of printable elements, the wafercomprising: a substrate; and a plurality of printable elements native tothe substrate, wherein each of the printable elements is at leastpartially released from the substrate and the printable elements areeach connected to the substrate by one or more alignment maintainingelements; and wherein the one or more alignment maintaining elementscomprise two alignment maintaining elements disposed on opposite sidesof the printable element.
 17. A wafer of printable elements, the wafercomprising: a substrate; and a plurality of printable elements native tothe substrate, wherein each of the printable elements is at leastpartially released from the substrate and the printable elements areeach connected to the substrate by one or more alignment maintainingelements; and wherein each of the one or more alignment maintainingelements is vertically connected between the printable element and thesubstrate in a direction orthogonal to a surface of the substrate onwhich the plurality of printable elements is disposed.